What is the correct way to allow for lateral movement of a timing pulley in a transmission system?

For example, there are four pulleys, one of which has to move up and down.

Is there standard practice for this sort of arrangement? Or an alternative that would allow the the large central pulley to move up or down.

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  • 2
    $\begingroup$ Unless you're considering that the belt is under tension and stretches, wouldn't you need at least one other pulley to move as well, to counteract the difference is necessary belt-length? $\endgroup$
    – Wasabi
    Mar 10, 2016 at 10:28

1 Answer 1


It is common for timing belt systems to have a tensioner pulley, effectively an idler mounted on a sprung radial arm. In most situation this is just to take up slack from wear and stretch and generally ensure consistent tension throughout its life but if you allow enough travel it could certainly achieve what you want.

Depending on the amount of travel you require you might need a constant force spring to get best results and you would need to consider whether radial or liner travel for the idler/tensioner is best.

It may even be that there is an off the shelf component that you can just bolt into the system.

  • $\begingroup$ An off the shelf component is what I was looking for. I thought it would be a not uncommon thing. Belt driven systems are fun though. $\endgroup$
    – oorst
    Mar 10, 2016 at 20:02
  • 1
    $\begingroup$ Look for a 'timing belt tensioner/idler pulley'. Often they are a complete assembly including pulley, bearing and sprung arm which bolts on to a mounting point on the block. $\endgroup$ Mar 10, 2016 at 20:09

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