Hydraulic tappet From Wikipedia, the free encyclopedia [hide hide]This ]This article has multiple issues. Please help improve it or discuss these issues on the talk page.
This article possibly contains original research . (August 2007) This article needs additional citations for verification . (November 2014) A hydraulic valve lifter , also known as a hydraulic tappet or a hydraulic lash adjuster , is a device for maintaining zero valve clearance in an internal combustion engine.. onventional solid valve lifters re!uired regular ad"usting to maintain a small engine clearance between the valve and its rocker or cam follower . This space allowed for thermal e#pansion, and prevented the parts from binding. This clearance space meant noisy operation and earlier wear, as the parts would rattle against one another until the parts heated up and e#panded. The hydraulic lifter was designed to compensate for this tolerance, allowing the valve train to operate with zero clearance$leading to !uieter operation, longer engine life, and eliminating the need for periodic ad"ustment of valve clearance. The hydraulic lifter, situated between the camshaft and each engine%s valve, is a hollow steel cylinder encasing an internal piston. This piston is held at the outer limit of its travel with a strong spring. The lobed camshaft rhythmically presses against the lifter, which transmits the motion to the engine valve one o ne of two ways& '. through through a pushrod pushrod which which actuates actuates the valve valve via a rocker rocker mechanism( mechanism( or or ). in the case case of overhea overhead d camshafts, camshafts, via direct direct contact contact with with the valve valve stem. stem. *il under constant pressure is supplied to the lifter via an oil channel, through a small hole in the lifter body. When the engine valve is closed +lifter in a neutral position, the lifter is free to fill with oil. As the camshaft lobe enters the lift phase of its travel, it compresses the lifter piston, and a valve shuts the oil inlet. *il is nearly incompressible, so this greater pressure renders the lifter effectively solid during the lift phase. As the camshaft lobe travels through its ape#, the load is reduced on the lifter piston, and the internal spring returns the piston to its neutral state so the lifter can refill with oil. This small range of travel in the lifter%s piston is enough to allow the elimination of the constant lash ad"ustment.
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' -ydraulic lifter preload
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) -istory
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/ Advantages
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0 1isadvantages
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4 5eferences
Hydraulic lifter pre-load The basic idea is to set the lifter plunger to the center of its travel. That gives the most leeway for temperature e#pansion6contraction. The amount of turns needed to set the plunger in the middle of its travel will vary from engine to engine because of differences in the thread pitch, rocker ratio, and lifter design.7'8 *ne approach is to use an identical lifter to the ones being installed that has been made 9solid9. :aking a lifter solid is a process where the lifter piston is prevented from moving this can be done with small welds made to the lifter, or by disassembling the lifter, removing the internal spring, and installing washers to take up the space and prevent the lifter piston from compressing. ;ome lifter manufacturers also sell solid versions of the hydraulic lifter being installed. The mechanic installs the solid lifter and rotates the engine crank until the cam is on the 9base circle9 +not on any intake or e#haust cam lobe. An ad"ustablelength pushrod checking tool is then installed on the valve location being checked. The pushrod length checking tool is e#tended until there is no slop +zero lash. Then, the check ing tool is removed and measured. The mechanic then adds .#ample& Ad"ustable push rod at zero lash ? =.<9. 1esired preload ? .<2<9. =.<9 @ .<2<9 ? =.<2<9 pushrods should be installed.
History The first firm to include hydraulic lifters in its design was the adillac '4 engine +:odel 02) first offered in '=/<. -ydraulic lifters were popular on automobiles designed in the '=B
Advantages As the whole process is actuated by hydraulic pressure at engine start, there is no need for service or ad"ustment. Another advantage is cheaper operation, as there is no need for service and charges associated with tappet maintenance. Csually hydraulic tappets survive through the whole of the engine life without any service re!uirements.
isadvantages There are a number of potential problems with hydraulic lifters. Fre!uently, the valvetrain will rattle loudly on startup due to oil draining from the lifters when the vehicle is parked. This is not considered significant provided the noise disappears within a couple of minutes( typically it usually lasts only a second or two. A rattle that does not go away can indicate a blocked oil feed, or that one or more of the lifters has collapsed due to wear and is no longer opening its valve fully. The affected lifter should be replaced in the latter situation. -ydraulic tappets re!uire more comple# and more e#pensive cylinder head design. A number of subcompact car manufacturers prefer solid valve lifters due to lower design and production cost rather than hydraulic tappets. Denerally, hydraulic tappets are more sensitive to engine oil !uality and fre!uency of oil changes, as carbon sludge and residues may easily lock up the tappets or block oil channels, making the clearance setting ineffective. This has negative impact especially on the engine camshaft and valves due to e#cessive wear if the clearance setting is not working correctly. As mentioned, one may avoid this by using the manufacturer recommended grade of engine oil, and by not e#ceeding the prescribed oil change interval. Et is a myth that in certain circumstances, a lifter can 9pump up9 and create negative valve clearance. The engine oil pump cannot generate enough pressure to cause 9pumpup9. The problem is due to weak valve springs which permit float at high engine speeds. The followers attempt to take up what they see as e#tra clearance. As this speed is maintained, the lifter will continue to e#pand until the valve is held off its seat when it should be closed. :aintenance of the valve springs at the correct strength is therefore very important to avoid engine damage.
!otes Tappets should be fitted while FC of oil6diesel li!uid& the originale!uipment manufacturers specify that the installer should be unable to compress them G>F*5> re fitting, given that these components are intended to take up the slack in the valve train. The reason they tap when faulty is because they cannot keep the gap correctly.
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