Vari-Speed Transmission

Discussion in 'Mechanic and Repair' started by AA961JESTER, Dec 28, 2001.

  1. AA961JESTER

    AA961JESTER LawnSite Member
    Messages: 48

    I recently picked up an older Bobcat (M4812ka) with the 6" casters. The vari-speed is not working. Can someone give me a rundown on how the system works. I can't figure out the pulley with the spring on it. I know this is the Vari-speed pulley, but how does it work?
  2. Grassman

    Grassman LawnSite Member
    Messages: 203

    I'm not exactly sure what your having the problem with. Sometimes the original pulley on the gearbox is replaced with a larger(speed pulley) , and it renders the selector useless because the belt is larger. Please explain a little more. Russ
  3. sdwally

    sdwally LawnSite Senior Member
    Messages: 386

    Most belt variable speed drive system operate in the same manner. One pulley with the large spring presses in on one side of the pulley to maintain proper belt tension. This side of the pulley should be able to move in and out freely with no spring tension.
    The other pulley would contain an mechanical or electrical/mechanical mechanism to move one side of the pulley in or out. When the two pulleys are conected with a belt you have a variable drive system.
    The drive pulley(one without large spring) is either operated by a lever, electrical switch, or centrifical force. At idle or low speed the belt will ride near the shaft of the pulley on the drive pulley and the belt will ride near the outside of the driven pulley. While the engine is running you can move the lever or speed up the engine(if centrifical) to adjust where the belt rides on the pulley without the spring. The pulley with the spring will self adjust to keep proper belt tension.
    It works similar to the gear shifts on ten speed bikes, however this system uses a belt and has no idler pulley.
    I hope this helps and didn't muddy the water anymore.
  4. AA961JESTER

    AA961JESTER LawnSite Member
    Messages: 48

    To clear things up. When I bought the Bobcat the lever for the vari-speed was there however the rod that goes to the Bell crank was gone, and the Bell crank was broken. The kid I bought it from replaced the belt with a shorted one for a single speed operation. The mower could be slowed through the use of the throttle but that would slow down the blade speed also. I have a new rod, bell crank, and belt on order. I am looking to gain information on how this system works, I am unfamiliar with it and dont understand the spring pulley thing. I am used to transmissions.

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