Screw jack design 1. Power Screws
Power screws are used to convert rotary motion in to translational motion . It is also called translational screw. They find use in machines such as universal tensile testing machines, machine tools, automotive jacks, vises; aircraft flap extenders, trench braces, linear actuators, adjustable floor posts, micrometers, and C-clamps. There are two kinds of power screws, hydraulic and mechanical power screws. A special case is screw jack which raises or lowers the load by applying a small force in the horizontal plane. A screw thread is formed by cutting a continuous helical groove around the cylinder. These grooves are cut either left hand or right hand. The majority of screws are tightened by clockwise rotation, which is termed a right-hand thread . Screws with left-hand threads are used in exceptional cases. For example, anticlockwise forces are applied to the screw (which would work to undo a right-hand thread), a left-hand-threaded screw would be an appropriate choice. Power screws are typically made from carbon steel, alloy steel, or stainless steel and they are usually used with bronze, plastic, or steel mating nuts. Bronze and plastic nuts are popular for higher duty applications and they provide low coefficients of friction for minimizing drive torques. There are important terms and figures that need to be understood before designing power screws:
1. Pitch: is the distance from a point on one thread to the corresponding thread on the next adjacent thread, measured parallel to the axial plane. 2. Lead: is the distance the screw would advance relative to the nut in one rotation. For single thread screw, lead is equal to pitch. 3. Helix Angle: is related to the lead and the mean radius by the equation below;
Figure
1: Common screw assembly
There are 3 types of screw threads used in power screws: 1. Square threads: a. Is used for power transmission tra nsmission in either direction b. Results in maximum efficiency and minimum c. It is employed in screw jacks and clamps 2. Acme threads: a. It is a modification modification of square thread b. Efficiency is lower than square threads c. The slope increases the area for shear d. It is easily manufactured
3. Buttress Thread: a. It is used when large forces act along the screw axis in one direction only. b. It has higher efficiency like square threads and ease of cutting like acme threads c. It is the most strong thread of all d. It has limited use of power transmission
Figure
2: Square thread
Figure
3: Acme threads
Figure
4: Buttress thread
2. Screw Jack Design Procedure 2.1. Design Tools
1. Books 2. Websites 3. Stationary 4. Calculator 5. Laptop 6. Printer 2.2. Design Objective State
the problem and clarify what is expected e xpected from the design Specify design considerations such as factor of safety, material selection criteria, and etc. To study effects of stresses on the power screw parts o D irect tensile or compressive stress due to axial load t he minimum cross section of the screw by the o Torsional shear stress in the twisting moment o Shear stress at the threads of the screw at the room diameter and at the threads of the nut at the outside o utside diameter due to axial loading o Bearing pressure at the thread surfaces of the screw and nut
To
determine the torque required to raise or lower the given load To determine the efficiency of the power screw To determine the dimensions of the different parts of the screw 2.3. Problem Statement
A mechanical power screw that can raise or lower 7 tons or 68.670 KN of load is intended to be designed. Different parts of the assembly such as the screw, the nut and the handle will be designed in an efficient and cost effective manner. 2.4. Design Considerations
1. Factor of safety for the assembly is taken 5 due to the nature of the design. Actually the factor of safety is taken 1.5 to 2 in static loading of ductile material. A higher factor of safety is considered due to the consequences of the failure. 2. Selection of Material for the screw and nut is of great importance. There are common materials used in the design of screw jacks like steel for the screw and cast iron, bronze or plastic for the nuts. Mild steel or hard steel is considered for different screw designs. In order to prevent friction cast iron or bronze is preferred for the design of the nut. Cup and frame are made of Grey cast iron which is cheap and has good mach inability. Material is selected as following: a. Screw:
Plane carbon steel (30C8 ± IS: 1570-1978) is selected because screw is always under Torsional, bending and axial load. Carbon steel is chosen due to the strength issues. This steel is also used for the handle of the screw jack. (
=
b. Nut:
400 MPa, _ =240 MPa, E=207GPa)
In order to reduce the friction resistance between the screw and nut a softer so fter material is selected for the nut. Phosphor P hosphor Bronze (Grade 1-IS: 28-1975) is a proper material for nut construction because it acts very well against wear resistance and reduces torque to overcome friction. = 190 MPa, _ yield (tension) =100 MPa, _ yield (compression) = ( 90 MPa, _ =80 MPa )
c. Screw Jack Handle:
Plane carbon steel (30C8 ± IS: 1570-1978) is selected s elected for the handle of the jack because of the high strength it offers. ( = 400 MPa, _ =240 MPa, E=207GPa)
d. Frame:
Grey cast iron is used which is cheap and has good mach inability. 3. The effective lifting height is chosen to be 0.5m (500 mm). 4. Average coefficient of friction between the material soft steel and cast iron is taken 0.10 when it is lubricated. But for this specific design, it is taken 0.18 assuming it dry for safe sa fe operations.(1) 5. Limiting values for bearing pressure between steel and cast iron is taken 15.05 MPa.(2) 6. According to agronomists the force of the hand is about 150 to 200 N. In this design we assume that is the handle is rotated by two hands which give 400 N hand forces for the design of the handle. 3. Calculations 3.1. Design of the screw
Procedure i. Core diameter of the screw is determined using allowable stress and the given load ii. Using the core diameter, the rest of the diameters and the pitch will be determined
from the table iii. Torque will be determined using the mean diameter, coefficient of friction and the pitch iv. Principle stresses due to the shear and compression stresses will be studied v. The dimensions for the screw is safe if and only if the maximum stresses are less than the allowable stresses
The next available diameter is 35 mm. For dc Table 17.2 (Normal series) we have
Pitch=7mm
The torque required to rotate the screw:
)
(2,3)
=35
mm, according to the
T=320N -m -m
Now, it is time to study principle stresses due to the combined stresses (compression and Torsional) and see if they are in limit for safe dimensions.
Principle stresses
Criteria for safe design against principle stresses
The design is therefore safe. 3.2. Design of the nut
Procedure i. Number of threads in engagement is found ii. Height of the nut is determined iii. Shear stress produced at the threads of the screw at the core diameter and at threads of the nut at the major diameter is studied. iv. For safe design, these shear stresses are compared with the allowable stresses
n=10.8
We take the number of threads n =12. The height of the nut is found from following equation:
The nut threads are subjected to crushing and shear. To check whether crushing is expected or not,
From the above result, crushing is not expected because crushing stress is much smaller than the bronze yield stress at compression.
From the result above, the nut threads are safe against shear stress. (13) =13.52MPa
The screws are safe s afe against the shear produced by the axial loading. 3.3. Design of the various diameters
Inner diameter of the nut collar co llar ( D1)
Outer diameter of the nut collar ( D2)
Thickness of the nut collar (t1)
Diameter
of head at the top of screwed rod ( D3)
Diameter
of pin which fits the cup c up ( D4)
3.4. Design of the handle
Where;
(21)
R3=radius of head R4=radius of Pin
Note: If the length of handle is too large, an alternative is to place the handle centrally and apply the force. Diameter
of handle
D=43mm
3.5. Buckling of the screw
(22)
Buckling is studied when the load is compressive and the unsupported length between the screw and the nut is long. When it is short, then it is assumed a column and buckling issue doesn¶t rise. If the critical load is more than the load we have then our design is safe and there is no chance of buckling.
There is no chance of buckling because the critical load is much greater than the design load which is 68.670kN. 3.6. Design of the body
Height of the head
Diameter
of the body at the t he top ( D5)
Thickness of the body (t3)
Inside diameter at the bottom ( D6)
Inside diameter at the bottom ( D7)
Thickness of the base (t2)
Height if the body =Max.
Lift Height+height of the nut +150mm =500mm+84mm+150mm =734mm 3.7. System Efficiency
If the screw friction fr iction and collar is friction is neglected, the efficiency of the system is calculated as below:
Atypical Screw Jack
