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Encyclopedia > Pushrods

A pushrod or overhead valve (OHV) type piston engine places the camshaft below the pistons and uses pushrods or "rods" to actuate lifters or tappets above the cylinder head to actuate the valves. This contrasts with an overhead cam (OHC) design which places the camshafts above the cylinder head and drives the valves directly.

Pushrod engines are somewhat maligned as "old fashioned" by the modern automotive press, but pushrod designs have been quite successful. Two specific problems present themselves with pushrod engines:

  • Limited RPM - Pushrods add complexity and have historically had a tendency to snap at high RPM. Therefore, conventional wisdom says that a pushrod engine cannot rev as high as an OHC design. However, modern manufacturing and materials has all but eliminated this area of concern. Modern pushrod engines can rev well past 6,000 RPM, which is about as fast as most passenger car engines ever run.
  • Limited valve flexibility - The biggest benefit of an OHC design is the use of multiple intake and exhaust valves, and Variable Valve Timing. No modern pushrod engine has had more than two valves per cylinder, while many OHC engines have used three, four or even five valves per cylinder to achieve greater efficiency. Rumors persist that General Motors is working on a 4-valve pushrod version of the LS2 V8, but this has yet to be seen.

  Results from FactBites:
Pushrods and Other Control Surface Linkages from Hobby Lobby! (512 words)
Two 36" long complete pushrods that use tiny 1/32" music wire inside special slippery plastic tube, which is inside a larger plastic tube.
It can be used for such linkages as a single pushrod from a servo connected to split elevators, or to flaps, or from dual servos to a boat rudder.
Use one for rudder pushrod, one for elevator.
Redundant pushrod-to-bellcrank connection - Patent 4030378 (1143 words)
A pushrod-to-bellcrank attachment includes a primary pivot connection and two secondary pin and slot connections so arranged relative to each other that there is a continuing operative connection of the pushrod to the bellcrank in the event the primary connection is lost by small arms fire.
This invention is particularly concerned with the attachment of pushrod 22 to bellcrank 10 which provides an improved redundant pivotal connection between these two members capable of continuing the same precise movements of the members following loss of the primary pivot between these members by projectile damage.
Pushrod 22 has a bifurcated end 28 the extremities of which are enlarged in area to form two side plates 30 which somewhat conform in shape to fin portion 20 of bellcrank 10 and receive portion 20 between them.
  More results at FactBites »



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