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THE OPE N POLYTECHNIC OFNEW ZEALAND
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Helicopter Transmission Systems 555—3—7
V’ CONTENTS Transmission Systems Transmission Components
Clutches \»,
Engine-driven Clutch Freewheel Unit
Gearboxes Types of Gears Used Main Rotor Gearbox (Single-stage Reduction)
Main Rotor Gearbox (Two-stage Reduction) Tail Rotor Gearbox Driveshafts Main Driveshaft Tail Rotor Driveshaft Driveshaft Couplings Other Components Intermediate Gearbox
Universal Joint Rotor Brake Oil Cooler Fan Unit
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AIRCRAFT ENGENEERKNG HELICOPTERS
ASSIGNMENT 7 TRANSMISSION SYSTEMS
The arrangement of drive shafts, gearboxes, and clutches that transmit the power developed by the engine to the rotor blades is often called the transmission or transmission system and, sometimes, the power train.
In this assignment, we shall discuss the
transmission components of various types of helicopter generally used in New Zealand. The type and location of the engine determines the transmission components used in the power train.
The smaller piston—engined
machines have the engine mounted immediately underneath the main rotor drive shaft, which is approximately the mid-point of the centre-of~gravity range of movement. Fuel is carried in saddle
tanks (Hughes 269 and Bell 47 e and J series) or within the fuselage immediately below the engine (Hiller UH 12 E series). Passengers and pilot are carried in a small cabin forward of the engine, and cargo is carried on external racks or on a cargo hook underneath the fuselage at the ideal c. of g. position.
This arrangement means
that the centre-of—gravity movement due to fuel usage is kept to a minimum and the helicopter itself has a compact shape. The large piston—engined machines had the engine mounted in
the nose, the main rotor gearbox mounted above the passenger cabin, the fuel carried in a bath—tub section under the passenger cabin, and the pilot and copilot carried in a separate cockpit sited above and behind the engine.
This arrangement provided a large cabin for
passengers and cargo and kept the centre—of—gravity movement due to fuel usage to a minimum.
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_ 2 _ The gas turbine engine, because of its smaller size and mass, is usually mounted on or gearbox, and the fuel is cabin. This arrangement seating position for the
near the cabin roof close to the main rotor carried in a bathtub section beneath the gives a compact helicopter and a very good pilot.
Figure l shows the power—plant locations for four types of helicopter used in New Zealand.
Note how the relatively heavy
piston engines are installed equally about the centre line of the main rotor driveshaft. The much lighter turbine engines are positioned so that roomy cabin and baggage spaces can be dispersed evenly about the mid centre of gravity position. This gives greater passenger comfort and easier cargo loading.
TRANSMISSION COMPONENTS Each helicopter manufacturer designs components to suit his own helicopter and, while the function and operating principle of a component will be the same from one type of helicopter to another, its constructional details are usually very different. In the following pages, we shall describe the components of the power train, using, where needed, two varieties of each component as examples. The essential components of a transmission are l.
An engine-driven clutch (on piston-engine helicopters),
2.
A freewheel unit,
3.
A main rotor gearbox,
H.
A tail rotor gearbox, and
5.
Driveshafts from the engine to the main rotor gearbox and from the main rotor gearbox to the tail rotor gearbox.
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_ 4 The engine-driven clutch is necessary only on piston-engined helicopters and, rarely used now, on turbine-engined helicopters, which employ a direct mechanical drive between the compressor and the helicopter transmission. The clutch is fitted to allow the engine to start without turning the transmission at the same time. Two basic types of clutch are used.
The first type engages
automatically once the engine has reached a low rev/min, and the
second type is controlled by the pilot, who can select clutch engagement or disengagement as he wishes.
Advantages of the second
type are that the engine can be started and warmed up and passengers can enter and leave the helicopter with the engine running and without the rotors turning. A freewheel unit, also called a one-way sprag clutch or,
simply, the sprag clutch, is fitted between the engine (or the engine and clutch combination) and the rest of the transmission. See Fig. 2. It allows the main rotor to turn at a higher equivalent rev/min than the engine. This condition occurs during autorotation or when power is substantially reduced for a steep approach to a landing site. If this unit were not fitted, the main rotor would try to drive the engine_during autorotation, which would cause a rapid loss of main rotor rev/min and a very high and uncontrollable rate of descent. The main rotor gearbox OP main transmission reduces the engine
rev/min to the low rev/min needed for main rotor head and blades assembly. external drive for the tail rotor and generator or alternator, an hydraulic generator. The other components that
efficient operation of the This gearbox often has an drive—mount pads for a pump, and a tachometer may be driven from this
gearbox are an engine cooling fan, for piston engines, and an oil cooler fan for cooling the lubricating oil, which is circulated through the gearbox by its own oil pump.
The carcass of this
gearbox is often used to carry flight loads from the main rotor
head into the airframe structure and to provide rigid attachment points for servo control units.
The gearbox is vented to the
atmosphere.
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_ 5 _ The tail rotor gearbox changes the direction and rev/min of the tail rotor driveshaft to suit the requirements of the tail rotor.
This gearbox consists of an input and an output gear,
meshing in an oil bath inside a sealed casing, which is vented to atmosphere. The tail rotor assembly is mounted on the tail rotor driveshaft. The casings of this gearbox carry flight loads from the tail rotor into the airframe structure and provide attachments for the tail rotor pitch—change mechanism. é
Driveshafts run from the engine to the main rotor gearbox and from the main rotor gearbox to the tail rotor gearbox.
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Alternatively shafts may run directly from the engine to both the main rotor gearbox and to tail rotor gearbox.
However the
system is laid out, the main rotor is always mechanically connected to the tail rotor so that one rotor cannot turn without the other turning. The drive shafting consists of aluminium alloy or steel tubing supported on grease-lubricated ball or roller bearings.
It is
connected to the gearboxes and the engine by drive couplings, which provide for some degree of malalignment due to movement of the engine on its flexible mounts and for the twisting and expansion and contraction of the airframe. Some further transmission components that may be found in helicopters are l.
An intermediate gearbox between the main rotor gearbox and the tail rotor gearbox,
2.
A universal joint between the two gearboxes,
3.
A rotor brake,
M.
An oil cooling fan unit, and
5.
An engine cooling fan used on piston—engined helicopters.
The intermediate gearbox, placed in the run of the drive
shafting between the main rotor gearbox and the tail rotorbox, changes the direction of the drive to the tail rotor. The gearbox consists of an input and an output gear meshing in an oil bath inside a sealed casing, which is vented to atmosphere. 555/8/7
_ 5 _ The universal joint, or Hardy Spicer coupling,
is used,
like
an intermediate gearbox, to change the direction of the drive between the main and tail rotor gearboxes. This joint is not used in the larger helicopters or where the change in direction is large. The rotor brake, is usually fitted close to the main rotor gearbox on the drive shafting to the tail rotor gearbox.
It provides
a means of quickly stopping the rotors after the engine has been shut down. The oil cooling fan unit, sited close to the main rotor gearbox, is driven by the main driveshaft or the tail rotor driveshaft.
This component provides a means of cooling the main
rotor gearbox lubricating oil.
The engine cooling fan, used on piston-enginedhelicopters, supplies air to cool the engine and the engine oil cooler.
It can
be driven directly by the engine or from the main transmission. The drive to this component is arranged so that it turns and
delivers air whenever the engine is running.
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Power trains
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SUMMARY A clutch must be fitted if the engine is mechanically connected to the power train. Thus, §§§_piston— enginedhelicopters have a clutch, but gas turbine engines usually do not. A freewheel unit is fitted in all power trains so that the engine can turn more slowly than the main rotor. This ensures that the main rotor cannot drive the engine. Gearboxes are used to change the direction of the drive, to speed it up, and to slow it down to suit the needs of
the main and tail rotors. Ancillaries can be driven from the main rotor box and from the driveshafts.
PRACTICE EXERCISE A
State whether each of the following statements is true or false: l.
A freewheel unit allows a piston engine to be started without all of the power train turning.
2.
The main rotor gearbox increases the rev/min of the engine to suit the requirement of the main rotor. .
3.
"Sprag clutcH'is another name for a freewheel unit.
4.
The only drives taken off the main rotor gearbox are the rotor tachometer and the tail rotor.
5.
To provide a quick warm up, the engine cooling fan turns only when the main rotor turns.
6.
A clutch is fitted to allow the engine to be started without the transmission being turned at
the same time. 7.
An intermediate gearbox is situated between the engine and the main rotor gearbox.
8.
The tail rotor always turns when the main rotor turns.
9.
The freewheel unit is fitted in the power train between the engine and the main rotor gearbox.
l0.
Drive couplings provide a rigid connection between the drive shafts and gearboxes. (Answers on page 43) 555/3/7
_ 9 _
CLUTCHES
Engine-driven Clutch Two main types of engine-driven clutch are used. l.
The centrifugal, and
2.
The belt drive.
They are
The large piston-engined helicopters having powers of 600 b.h.p. and upwards used an oil-operated hydro-mechanical clutch
very similar to an automotive fluid flywheel.
The lower—powered
piston-engined helicopters use either the centrifugal or the belt-
type clutch, the belt type being particularly reliable and trouble free. Centrifugal clutch:
As the name suggests, the clutch is
operated by centrifugal force.
Splined to a drive ring bolted to
the engine crankshaft is a spider, and freely pivoting on this spider are four massive steel shoes with brake lining material riveted to their outer surfaces. On the input shaft of the main rotor gearbox is splined and secured a sturdy steel clutch drum, which encircles the spider and shoes assembly. Immediately the engine is turned, the shoes are flung outward by centrifugal force,
contact the clutch drum, and slowly start to turn the main and tail rotors. As the engine fires and its rev/min build up, the shoes are held more firmly against the clutch drum until, at about
70% engine rev/min, the clutch drum rev/min become nearly the same as those of the engine.
No further increase in engine rev/min
will decrease the remaining slippage.
The engine rev/min are
now reduced for a moment and then slowly increased and, from the
time the engine rev/min equal those of the clutch drum, no further slippage takes place and the drive between the engine and the rest of the transmission system is complete.
The reason for the initial constant slippage is that the coefficient of sliding (kinetic) friction is slightly less than that of static friction. Once the shoes are slipping, as they must at initial startup, they will want to keep slipping. when we reduce the engine rev/min below that of the clutch drum and then
555/3/7
_ 10 l restore them, a time comes when both rev/min are the clutch drum slowing down due to friction and drag in transmission and the engine rev/min increasing). At the higher value of static friction takes effect and occurs.
same (the the this moment, no more slippage
Figure 3 shows a centrifugal clutch assembly.
The drive
from the engine is through the splines(1HJ of the spider (5), the four heavy clutch shoes (8), and into the drum (l). The splines (13) of the drum engage on the splines of the transmission lower sun gear, and the complete clutch assembly is secured to this shaft by a large nut.
The hearing (15) locates and supports the
spider centrally in the drum.
1. Clutch drum
2. 3. 4. 5. 6. 7. 8. 9. 10. 11.
Bolt - bearing retainer Retainer Bearing Spider Lock ring Pivot pin Clutch shoe assy Bushing Shoe Plate
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13. Clutch drum drive splines 14. Spider drive splines 15. Drum - spider bearing
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_ 11 _ This clutch is sprayed with engine oil to cool the finned drum and to lubricate the clutch shoe pivots and the drum-spider bearing. The oil supply is part of the transmission lubrication system and is tapped from the engine lubrication system. It flows all the time that the engine is running. Belt clutch:
This type of clutch consists of a driving
pulley connected to a driven pulley by a slack belt or belts. To make a positive drive between the two pulleys, the slack belt is tensioned by a lever-operated jockey pr idler pulley.
This
type of drive is mechanically efficient, and if the diameters of the driving and driven pulleys are different, a reduction or an increase in rev/min of the driven pulley can be effected. The belt clutch used on one type of helicopter has eight belts to take the drive from the engine-driven pulley to the
slightly larger diameter~driven pulley so that the first stage of reducing the engine rev/min to those needed by the rotor head is made in the clutch. The belts are tensioned by a pilot-controlled linear actuator connected to the idler pulley arm by a cable and spring assembly, the final tension on the belts being determined by the tension applied through a cable_and spring assembly. The linear actuator is electrically driven and controlled by a three~ way guarded switch in the cockpit marked engage, hold, and release, and its travel is determined by two internal limit switches.
The
wiring circuit is arranged so that the engine starter motor cannot be energised with the clutch engaged, as could happen if the engine
had been shut down with the clutch engaged. Figure H shows a belt—driven clutch assembly with its eight matched V belts.
When the clutch is engaged, the linear actuator
contracts and pulls the idler pulley assembly downward, compressing
the springs in the clutch-control spring assembly.
These springs
determine the pressure exerted by the idler pulley on the V belt
set.
when the clutch is released, the linear actuator extends
and tension comes off the clutch control spring assembly.
To help
the idler pulley remove its pressure on the V belt set, a return spring is fitted to its pivot shaft.
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All three pulleys are mounted on grease-packed heavy-duty ball bearings, and the upper pulley contains, within its hub, a sprag clutch (freewheel unit).
The belt-drive frame is steadied
at its bottom end by a strut assembly from the airframe, and
the complete belt drive assembly is enclosed by a detachable guard assembly. Any heat generated during clutch engagement and during engaged running is dissipated by the free movement of air
within the guard assembly.
Freewheel Unit Two types of freewheel unit are used. l.
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The cam and roller bearing type, usually called a freewheel unit,and
2.
The sprag type, called a sprag clutch.
The word sprag means "a device to prevent a vehicle from running backward". This suggests that a sprag will allow forward movement but will lock and prevent rearward movement. Figure 5 shows the arrangement of driving member, driven member, and sprags in their retainer. Engine power is applied to the driving member, which causes the sprags to try to roll to an upright position. This action locks the driving and driven members together and completes the drive from the engine to the transmission.
To help the sprags in this action, a circular spring
is fitted to hold them in light contact with the driving and driven members. When the engine is throttled back, the driving member immediately slows down and the driven member turns faster than the driving member.
The sprags roll away from their locking
contact with the two members, and the transmission freewheels.
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Figure 6 shows a cam-and-roller type freewheel unit in diagram form.
The driven member has a series of cams cut on its
outer surface, and the driving member has a smooth inner surface. Between the two is a set of rollers with one roller to each cam. Engine power is applied to the driving member, which, as it turns, carries the rollers forward until they lock by wedging between the cams and the driving member‘s inner surface. When the engine is throttled back, the driving member immediately slows down, the rollers run down the cams to unlock the driving and driven members, and the transmission freewheels.
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_ 15 _ Figure 7 shows a sprag clutch assembly and its fitment into a helicopter transmission.
In this installation, the sprag clutch
forms the hub of the belt—driven clutch upper pulley and makes a compact and easy—to—service assembly. This unit functions by the metal-to-metal contact of the
sprags with the driving and driven members.
To prevent damage to
these parts during freewheeling, that is, when they are sliding over each other, the unit is filled with an oil, which is checked for quantity and changed at intervals specified by the helicopter
manufacturer.
The aft cap has removable sealed bolts, which
provide access into the unit for oil filling and draining. In the sectional view shown in Fig.
8, the drive comes from
the drive belts into the upper pulley, through the driving and
driven members of the sprag clutch, and into the input pinion shaft. The input pinion shaft is part of the main rotor gearbox, and the tail rotor driveshaft is splined on the aft end of this shaft. Thus, the tail rotor driveshaft is directly connected to the main rotor gearbox, and the main and tail rotors can turn with the upper pulley and engine stationary. %§§ _
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-17.. The cam-and-roller type freewheel unit shown in part
section in Fig. 9 takes the drive from the engine through the flanged ring gear (l), the drive head and gear wheel (2)
the 5
rollers (M), the driven cam (5) and, finally, on to the stub shaft (6).
The unit is connected between the stub shaft (6) and
the main rotor gearbox by two flexible couplings and a robust drive shaft.
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555/3/7
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SUMMARY The two types of clutch in general use are l.
The centrifugal type, and
2.
The belt type.
They are usually needed by and used with piston engines only. The two types of freewheel unit in use are l.
The cam—and—roller type, and
2.
The sprag type.
Both types perform the same function.
PRACTICE EXERCISE B
State whether each of the following statements is true or false: l.
The centrifugal clutch starts to engage immediately the engine fires up.
2.
When a belt-type clutch is engaged, the starter motor cannot be energised.
3.
A sprag clutch locks in a clockwise direction, and a cam and roller clutch locks in a counterclockwise direction.
4.
Both types of freewheel unit are lubricated with oil.
5.
Clutch—drive belts are unmatched and may be replaced one at a time.
6.
A centrifugal clutch is selected to engage with a linear actuator.
7.
Air is circulated around the belt-type clutch to dissipate heat.
8.
The idler pulley assembly applies tension to the drive belts.
9.
A centrifugal clutch may be disengaged by running the engine at idle RPM.
l0.
A sprag clutch performs the same function as a cam and roller clutch. (Answers on page 44)
_ 19 _
GEARBOXES
Types of Gears Used In helicopter gearboxes, three types of gears are widely used See Fig. l0 (a), (b), and (c).
The spiral bevel gear is always
used in preference to the straight bevel gear, and the spur gear
is used more often than the helical gear. Recently, a fourth type of gear has been developed by Westland Helicopters.
This is called a conformal gear, which is
combined with a helical pattern. See Fig. l0 (d), This gear will transmit very high powers, is quiet in operation and, because of the high strength of the tooth section, a larger reduction in speed can be had from a smaller driving gear than usual.
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_ 29 _
Main Rotor Gearbox (Single-stage Reduction) A simple, compact, single~stage reduction main rotor gearbox is shown in exploded form in Fig. ll.
It consists of a spiral bevel
input pinion (1) driving a spiral bevel ring gear (2) to give a speed reduction of approximately M.5 to l.
The ring gear is
carried on the drive shaft coupling (3), which is splined internally at its top end for the mating external splines of the main rotor drive shaft.
The driveshaft coupling is also splined internally
for the lower driveshaft (H) to the oil pump and tachometer drive (5).
The input pinion and driveshaft coupling assemblies are
mounted on heavy-duty tapered and plain roller bearings. This gearbox is completely self-contained, with all parts and the oil supply being housed in the upper and lower housings (6) and (7).
An oil pump (8) supplies oil at a low pressure to an oil
jet at the top of the upper housing. This jet sprays oil on to the driveshaft coupling upper bearing. Oil is also sprayed to the disengaging teeth of the input pinion and bevel ring gear to cool and lubricate the teeth and it is fed to the aft bearings of the input pinion to ensure positive lubrication and cooling of those heavily loaded bearings. Oil is added through a filler cap that incorporates a filter screen.
The oil level is checked by a ball-locked dipstick.
A magnetic drain plug and self-closing valve unit is installed in the lower housing.
-All oil must pass over the
magnetic plug, and so any magnetic material that may have been worn from the gears and bearings is deposited on the plug.
This
plug can be easily removed for inspection and cleaning. A removable fine mesh gauze filter is provided in the oil way to the input pinion aft bearing.
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the centre of the coupling assembly (3) to prevent debris from passing down the hollow main rotor driveshaft and into the gearbox.
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555/3/7
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Oil pressure and temperature are monitored by transmitting units, which control warning lights in the cockpit.
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gearbox assembly is vented to atmosphere through a special fitting
in the top of the upper housing. the assembly cool.
Air flow around the casings keeps
Figure 12 shows this gearbox assembled and in part section.
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Assembled gear box in part section
The gearbox is braced to the helicopter centre frame by three struts, and, through two shock mounts, it supports the driving end of the engine.
The main rotor mast and the main rotor driveshaft are not thought of as part of the main rotor gearbox. However, they join the gearbox to the main rotor head, so we shall discuss them briefly
555/3/7
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The main rotor mast is attached at its base to the upper
housing of the gear box.
At about its mid»height position, it is
secured by rivets to a cabin—section beam and by three support
struts to the centre frame.
See Fig. l3. TAIL ROTOR TRANSMISSION
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Gearbox and mast in relation to rest of transmission
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_ Qu _ The main rotor driveshaft is housed in the mast. At its lower end, it engages with the driveshaft coupling of the gearbox. At its upper end, it carries the main rotor head assembly.
This
driveshaft is held in the mast by a heavy-duty thrust bearing and thrust nut.
Lift and thrust forces in the rotor head pass from the rotating driveshaft and thrust bearing, through the thrust nut, into the stationary mast. The forces are then led through the support struts and gearbox struts into the centre frame. The complete assembly of gearbox and mast provides attachment
points for the cyclic and collective controls.
Main Rotor Gearbox (Two-stage Reduction) Figure 1% shows a transmission with a two-stage speed reduction. The first reduction occurs between the main input pinion (1) and the main input spiral bevel gear (2), with a reduction of about 3 to 1 being obtained. The second reduction is between the planetary assembly (8) and the planetary ring gear
(R), with a reduction of about 5 to l. The total reduction, the product of the two ratios, is about l5 to l.
The gear ratio of a planetary system may be found from the equation . Ratio
=
No. of teeth on sun gear + No. of teeth on ring—gear r r No. of teeth on sun gear
The drive from the engine arrives at the main input pinion adapter (5): and passes through the first stage reduction gears (1) and (2) to the main input gear shaft (6).
From the internal splines
in (6), the drive passes to the planetary sun gear (8), the planet gears and planetary ring gear (9) and (R), to the planetary spider (l0), and then through splines in (10) to the main rotor mast (ll).
555/3/7
Main input pinion Main input spiral bevel gear
Planetary assembly Planetary ring gear Adapter main input pinion
Main input gear shaft Support assembly, lower mast bearing
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_ 25 _ The main rotor mast is located at its lower end by the lower mast bearing support assembly (7) and is secured in the top case assembly (12) by the mast thrust bearing and seal assembly (13). Lift and thrust forces from the rotor head pass down the rotating mast through the thrust bearing (13), and into the top case (12). From there, the forces pass into the airframe through two pylon
support links that attach to each side of the main case assembly (15) ~— see position A for the left—hand pylon support attachment.
The gearbox can pivot fore-and-aft on the pylon supports. This movement is restrained by the drag pin assembly (17) of Fig.
15, which links the gearbox to a rubber isolation mount
attached to the fuselage. A positive mechanical restriction is provided for the gearbox movement if the isolation mount fails. This mounting arrangement tends to dampen or remove any residual vibration from the rotor head and thus provides a smooth flight for the airframe.
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The accessory drive gear (16) drives a small oil pump for the lubrication system. It also drives the rotor tachometer generator and the hydraulic pump that powers the servo controls used in the main and tail rotor controls. The hydraulic pump unit and the tachometer are attached to the external mount face of the oil pump (16). See Fig. 1M for parts list.
Figure 15 shows the assembled gearbox in part section. It is clearly a compact and simple unit.
This gearbox has its own lubrication system, which it shares with the freewheel unit mounted on the engine accessory case. system,
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schematically in Fig. 16, consists of a simple
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_ 29,The oil is cooled in a thermostatically controlled oil cooler, which is supplied with cooling air by a fan unit driven
by the tail rotor driveshaft. Drain plugs, magnetic or electric chip detectors, temperature and warning switches, inlet and outlet filters, and an adjustable
oil pressure relief valve are used to control and monitor the system.
Oil is added to the system through a combined filler cap, breather, and strainer assembly, and the oil quantity is easily seen through the oil-level sight glass. Figure 17 shows the main gearbox in relation to the rest of the power train.
In this example, the drive from the freewheel
unit goes forward into the main gearbox and aft to the tail rotor
gearbox. Both drives are connected mechanically at the freewheel unit so that, when the main rotor turns, so does the tail rotor.
Tail Rotor Gearbox The tail rotor gearbox is a simple gearbox with an input . gear and an output gear.
A speed reduction/increase may be
arranged between the two gears to suit the need of the tail rotor. The gearbox is usually mounted, with no vibration-absorbing devices, directly on to the aft end of the airframe.
555/3/7
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Breather - filler assembly Liquid level plug Housing assembly Attachment points (four) Drain valve and chip detector
Tail rotor gearbox
555/3/7
-31..
Figure 18 shows a tail rotor gearbox in exploded form.
The
drive from the tail rotor driveshaft passes through the driven
spline (l) into the input level gearshaft assembly (2). It then passes through the input and output bevel gears (3) and (M) to the output gearshaft (5). The input shaft assembly is supported on two ball bearings (6) and (7), and the output gearshaft is supported by a roller bearing (8) and a duplex ballthrust bearing assembly (9). The complete assembly is attached to the airframe by four studs at (13). The gearbox contains its own oil supply, the gears and bearings being splash-lubricated by oil being thrown around as
the gears turn.
The gearbox is filled through a breather/filler
assembly (10), with the oil level being easily seen through the liquid level plug window (ll).
The oil is cooled by the air flow
over the housing assembly (12).
A drain valve and magnetic chip
detector (lq) is provided at the bottom of the housing to trap any magnetic debris from the gears and bearings. This plug may be removed for inspection without the oil being drained.
SUMMARY
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Main rotor gearboxes of modern helicopters have their own oil supply and lubrication system. The main rotor gearbox will be used to drive at least r 5 i ‘
l.
A rotor tachometer, and
2.
An hydraulic pump.
Gearbox casings may be used to dissipate heat to circulating air. Magnetic plugs are used in all gearboxes to trap magnetic debris.
555/3/7
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- 32 _
PRACTICE EXERCISE C
State whether each of the following statements is true or false: l.
The oil level in a modern gearbox is checked by a
dipstick or through a sight glass. 2.
All main rotor gearboxes have at least two stages of speed reduction between the input and output shafts.
3.
A gearbox is vented to atmosphere to prevent a buildup of air pressure inside the casings.
4.
The number of teeth on the planet gears determines the speed reduction in a planetary gear system.
5.
The output gearshaft of a tail rotor gearbox must be able to accept thrust from two directions.
6.
A magnetic drain plug will trap all metallic particles in the lubricating oil.
7.
You can usually remove magnetic chip detectors from a gearbox without first draining the oil.
8.
A tail rotor gearbox must give a speed reduction to the tail rotor.
9.
A heavy duty thrust bearing is used to transfer the flight loads from the rotor head to a nonrotating part of the helicopter.
10.
The tail rotor gearbox is usually directly attached
to the airframe structure. (Answers on page 44)
DRIVESHAFTS Main Driveshaft The main driveshaft is a substantial but hollow shaft made from aluminium alloy or steel. It is attached at each end to the engine and main rotor gearbox by special couplings that allow some degree of malalignment due to flexing of the airframe and movement of the engine on its mounts. 555/3/7
_ 33 _ Some main driveshafts are balanced during manufacture, and extra care must be taken with these shafts to ensure that
they are fitted correctly.
Item 3 of Fig. l7 shows a main drive-
shaft with its couplings.
Tail Rotor Driveshaft This assembly may consist of either one long driveshaft and its support bearings or two or more shafts coupled together. These shafts may be made from steel or from aluminium alloy,
and because they transmit far less power than the main rotor driveshaft, they are of a much lighter construction.
At each end of
the shaft is a special coupling that allows for flexing of the airframe.
Because of the length of the shaft, it is supported at
regular intervals by bearings mounted in bearing hangers attached to the airframe. These bearings are lubricated with grease. Because correct alignment of this driveshaft is important to the life of the shaft and the bearings, provision is made for adjustment of the bearing hangers. Thus, the shaft may be moved in any direction as desired to obtain an accurate alignment between its input and output ends.
The shaft or shafts may be balanced,
and they may also have to be assembled with a particular angular
relationship to the main rotor gearbox and the tail rotor gearbox. Items(5)to(l2)of Fig.l7
show the assembly of the various
components of a tail rotor driveshaft.
Figure l9 shows in some detail the tail rotor driveshaft of a light helicopter.
This figure should be viewed with Fig. l7.
Note that item (9) of Fig. l7 becomes item (l) of Fig. 19.
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Tail rotor driveshaft assembly
555/3/7
_ 35 _ Driveshaft Couplings Couplings are used to connect the driveshafts to the engine, to the gearboxes, and to each other. The amount of power being transmitted determines the size of the coupling.
Whatever their size, all couplings have the ability to allow for some small degree of malalignment and relative movement between one transmission component and another. Three types of coupling in common use are l.
The rubber coupling,
2.
The Thomas, or disc, coupling, and
3.
The splined coupling.
These types are shown in Fig. 20, 2l, and 22.
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The rubber coupling consists of two metallic plates with vanes formed on one face of each plate.
The coupling is assembled
with the vaned faces towards each other and rubber blocks interposed between the vanes. The driving plate transmits the drive to the driven plate through the rubber blocks, which are bonded to the vanes. This type of coupling can be used to transmit high powers.
555/3/7
_ 35 _
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A Thomas, or disc, coupling
The Thomas, or disc coupling, consists of a pack of thin steel discs.
The steel used is made with its grain structure
running in parallel lines, which makes the steel easier to flex along the grain than at 90° to it. The pack is assembled with the grain of each disc at 90° to that of the disc above and below it. This ensures that flexing can take place evenly in all directions. The discs have a series of bolt holes in them and are secured to opposing flanges by alternate bolt holes. Thus, if a coupling has six bolt holes at 60° to each other, it will be secured by three bolts at l20° to each other to one flange and by the other three bolts to the other flange.
555/3/7
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A splined coupling
The splined coupling shown in Fig. 22 consists of a steel inner spherical coupling (3) bolted to the driveshaft (l). Surrounding the inner coupling (3) is the steel outer spherical coupling (Q), which is bolted to the drive flange of the engine or main gearbox.
The coupling is packed with grease and is sealed
by the plate (5), the seal (2), and two O rings. A spring is fitted inside the coupling to hold the driveshaft (l) in about _ its mid travel.
One of these couplings is fitted at each end of
the driveshaft.
This type of coupling is used to transmit high
powers.
Of the three types of coupling, the splined coupling permits most movement of the driveshaft, the shaft being able to slide in and out, tilt up and down, and move from side to side.
This
type of coupling is needed to connect the main driveshaft to a main rotor gearbox that is not rigidly attached to the airframe.
OTHER COMPONENTS Intermediate Gearbox This gearbox is used where an abrupt change in the direction
of a tail rotor drive is needed. Figure 23 shows an intermediate gearbox.
It consists of two
spiral bevel gears supported on heavy-duty roller bearings in a
housing containing a bath of oil.
As the shafts turn, oil is
thrown by the gears up the oil feed pipe to positively lubricate
the output shaft upper roller bearing. 555/3/7
The other bearings are
_ lubricated by splash oil.
38 _
Fins on the main housing dissipate
heat to atmosphere, and duct
stream air around the gearbox.
The assembly has an oil filter and breather, a drain plug, which can be a combined drain valve and magnetic chip detector, and an oil level window that is easily seen from outside the helicopter The gearbox is rugged l y b ui'lt and is firmly attached to a strengthened part of the airframe.
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Universal Joint The universal joint, or Hardy Spicer coupling, may be used in the tail rotor driveshafting where only a small change in the
direction of drive is needed.
It is an efficient substitute for
costly intermediate gearbox.
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Figure 2H shows a typical universal joint.
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These joints are used only in the smaller helicopters.
The Rotor Brake This component is not essential for the functioning of the helicopter transmission and, for many helicopters, it is supplied only as an optional.kit.
The rotor brake provides a safe method
of rapidly bringing the rotors to rest in an emergency.
It may
be very desirable for certain types of surveying and for the safety and peace of mind of passengers entering or leaving the helicopter.
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The rotor brake consists of an aircraft-type brake unit, a hydraulic accumulator or relief valve, a hand~operated master cylinder/reservoir assembly, and connecting flexible hoses and pipelines.
A rotor brake 0N warning light may be fitted to the
instrument panel.
The oil used is normally a mineral hydraulic
fluid.
The brake drum or brake disc is bolted to the main driveshaft or tail rotor driveshaft and is sited as close as possible to the main rotor gearbox. The housing containing the brake pads or brake shoes is bolted to a reinforced part of the airframe structure.
The capacity oi and the pressure in, the hydraulic
accumulator govern the maximum force that can be exerted by the master cylinder. Alternatively, a relief valve bleeding excess pressure back to the master cylinder is used. The master cylinder/ reservoir assembly is located within easy reach of the pilot and is usually operated by his left hand. Provision may be made for locking the rotor brake in its ON position.
011 Cooler Fan Unit Most main rotor gearboxes have their own lubricating oil systems, and an essential part of this system for all but the smallest helicopters is an oil cooler or a positive means of cooling the oil.
The oil cooler is needed because a large amount of heat is generated by the meshing gears. Unless this heat is removed, it will overheat metallic parts, destroy the gearbox oil seals and packings, and impair the quality of the lubricating oil.
The oil cooler on a fixed-wing aircraft has cooling air ducted through its matrix, the airflow being obtained from the forward speed of the aircraft. The helicopter, however, cannot rely on its forward speed to supply the airflow. AS when hgveringaitg airspeed is nil. To provide a cooling airflow, a fan driven off the main rotor gearbox or the tail rotor driveshaft is used.
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the helicopter is ducted to the fan and then from the fan to the oil cooler matrix. In some installations, the fan and cooler may be joined together as one assembly. In Fig. 1?, an oil cooling fan unit is shown as item (8) and consists of a squirrel cage type impeller driven by the tail rotor driveshaft. The engine oil cooler is mounted on the fan unit, and cooling air is ducted to the main gearbox oil cooler (l3) and around the finned hydraulic system reservoir.
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The oil cooler fan unit in Fig. 25 is belt driven off the main rotor driveshaft. Cooling air enters through two ram air scoops, is ducted around the main rotor gearbox casings, and passes into the fan unit. The fan unit outlet air is ducted through the engine oil cooler matrix and to various places around
the engine. The air flow over the main rotor gearbox casings keeps the gearbox at a safe working temperature. Because of the fan unit, the cooling air flows while the helicopter is hovering.
SUMMARY Driveshaft couplings allow for some malalignment between transmission components. An oil cooler fan unit is often used to supply the essential cooling air for transmission and engine oil cooling.
PRACTICE EXERCISE D
State whether each of the following statements is true or false: l.
The main purpose of an oil cooler fan unit is to supply cooling air for the engine and transmission oil systems.
2.
Tail rotor driveshaft bearings are lubricated with grease.
3.
Because of their strength, straight bevel gears are always used in place of spiral bevel gears.
4.
A rotor brake is fitted as close as possible to the engine.
5.
A Thomas coupling consists of a stack of thin steel discs.
6.
A splined coupling will permit very little axial movement of a driveshaft.
7.
A universal joint is normally used in the tail rotor driveshaft run when a small change in the direction of the drive is needed.
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8.
Of the Thomas, rubber, and splined couplings, only the splined coupling must be lubricated.
9.
An intermediate gearbox is used when the direction of rotation of the tail rotor driveshaft must be changed.
lO.
A rotor brake system has a device fitted to prevent excessive pressures being applied to the brake
disc. (Answers on page 44)
ANSWERS TO PRACTICE EXERCISES EXERCISE A
Statements 3, 6, 8, and 9 are true. l.
False. A freewheel unit is fitted to allow the power train to keep turning when the engine stops or slows down.
2.
False. The main rotor gearbox decreases the rev/min of the engine.
H.
False. Other components that may be driven by the main rotor gearbox are an alternator/generator and a hydraulic pump.
5.
False.
The engine cooling fan will turn whenever the
engine is running. 7.
False.
The intermediate gearbox is situated between
the main rotor gearbox and the tail rotor gearbox. l0.
False.
Drive couplings are designed to be fairly
flexible to allow for malalignment of power train components.
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_ nu _ EXERCISE B
Statements l, 2, H, 7, 8, and l0 are all True.
3.
False. Both types of clutch can be designed to lock in either direction.
5.
False. Clutch-drive belts are supplied as a matched set and must be installed as a matched set.
6.
False. A centrifugal clutch engages because of the rev/min of the engine. A linear actuator is used to tension the belts of a belt-drive clutch.
9.
False. With the engine at idle RPM, enough centrifugal force is generated to keep the clutch engaged. The only way to disengage a centrifugal clutch is to
stop the engine. EXERCISE C
Statements 1, 3, 5, 7, 9 and 10 are all True. 2.
False. Main rotor gearboxes can have one or more stages of speed reduction.
H.
False. The speed reduction is determined by the number of teeth on the sun and ring gears.
6.
False.
A magnetic drain plug will trap only magnetic
material.
8.
False. Depending upon the manufacturer, the tail rotor gearbox may give a reduction, an increase, or no change in rotational speed.
EXERCISE D
Statements l 3. False.
9
2
9
5
3
7
9
8 and l0 are all True '
Spiral bevel gears are always used because they
are stronger and quieter in operation.
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False. A rotor brake is fitted as close as possible to the main rotor gearbox.
6.
False. A splined coupling is the coupling used when axial movement of the driveshaft is needed.
9.
False.
An intermediate gearbox is used to change the
direction of the tail rotor drive.
TEST PAPER 7 A main rotor gearbox has its first stage of speed reduction through two spiral bevel gears with 25 and HO teeth respectively. Its second stage reduction is through a
planetary system whose sun gear has AH teeth, and ring gear, 1H3 teeth. Make a diagram of this gear train and calculate the total speed reduction.
Sketch, in reasonable proportions, a roller-and-cam~type freewheel unit. Show the power input going to the inner member 5 the power takeoff from the outer member 9 and the power being transmitted in a clockwise direction. State the differences in purpose and operation between an engine-driven centrifugal clutch and a one-way sprag clutch. What ancillary components could be driven by the main rotor
gearbox? What ancillary component must be driven by this gearbox? Give reasons for your answers. Sketch a driveshaft and coupling assembly that would be used to join an engine to a main rotor gearbox that rocks in a fore and aft direction. Give reasons for your choice of coupling.
Why must an oil cooler fan unit be fitted to most helicopters? State two reasons for having a rotor brake fitted in the power train.
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Draw a diagram of a power train for a piston-engined helicopter.
Include in your drawing
(a)
An intermediate gearbox,
(b)
An oil cooler fan unit, and
(c)
A rotor brake.
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