SER FAQ: LMFAQ: Crankshaft anatomy

S.E.R. FAQ

NotTaR of small Gasoline Engines and Rotary Lawn Mowers : Crankshaft anatomy
Copyright © 1994-2007, Samuel M. Goldwasser. All Rights Reserved. Reproduction of this document in whole or in part is permitted if both of the following conditions are satisfied: 1. This notice is included in its entirety at the beginning. 2. There is no charge except to cover the costs of copying. I may be contacted via the Sci.Electronics.Repair FAQ (www.repairfaq.org) Email Links Page.

<< Small engine technology | Index | Basic operation of a floa.. >>

Crankshaft anatomy

The diagram below shows the major parts of the crankshaft:

Main bearings support both ends and whatever is driven by the engine.
The crank pin journal is the surface upon which the piston's connecting rod bearing acts to rotate the crankshaft.
The crank gear drives the camgear/camshaft assembly which operates the valve lifters/valves and possibly the oil pump as well.
The flywheel is actually mounted on a machined taper (not shown) held on by a large nut. The starter engages a clutch mechanism which may also be mounted with the flywheel. For point-type ignition systems, a cam to operate the contact points is usually located on the crankshaft under the flywheel.
The blade or other driven equipment is mounted on the other end with an adapter (not shown) held on by another large nut.
Auxiliary equipment like front wheel drive may be powered from a worm gear which is part of the PTO end of the crankshaft (not shown).
The counterbalance weights help to smooth the operation of the engine.


                           Counterbalance weights
                                __   |   __
                               |  |  |  |  |
       Threads for             |  |<-+->|  |           Threads for
       Flywheel nut            |  |     |  |           blade adapter nut
             |                 |  |     |  |  __              |
             V     _________---|  |     |  |-|__|_______      V
            ...---/         |  |  |     |  | |__|       ----+...
            |||   |         |  |  |     |  | |__|       |   ||||
            '''---\_________|  |  |     |  | |__|_______----+'''  
                         ^  ---|  |_____|  |-|__|  ^          
                         |     |--|     |--|  ^    |
                Main bearing   |  |     |  |  |   Main bearing
               (Flywheel end)  |  |_____|  |  |  (PTO/blade end)
                               |__|  ^  |__|  |
                                     |        +--- Crank gear (meshes
                                                   with cam gear)
                                 Crank pin
                                  Journal

<<Small engine technology | ToC | Basic operation of a floa..>>