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This question has always occupied my mind that why we see crank gears with walk blockers but camshaft pulleys with walk blockers are rarely seen on most cars. Walk blockers on camshaft pulleys can prevent lots of catastrophic failures in case of improper setting of belt tension. They can prevent belts from walking off the pulleys. However, most manufacturers make cam pulleys without blockers. Why?

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Not that what Solar Mike is saying is wrong, but to add to his post ...

There's three problems with what you're suggesting:

  1. Some companies DO put the walk blocker on the camshaft pulley. Look at the Honda J35 V6 (goes for the other J3x series v6 engines as well) for instance:

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The rear cam has the walk blocker on it. This isn't a one off either. More than just Honda has done this.

  1. You only need them on one pulley to be effective. It doesn't matter which position they put the walk blocker at, it just needs to be on there. The width of the belt prevents much of this from happening as long as everything is aligned correctly (cam/crank gears, tensioner, idler, water pump, etc).

  2. The only real reason many have one in the first place is because they use plastic timing belt covers (like the J35). If you didn't have one there, the timing belt would eat through the cover if it tried to walk off one of the pulleys. If the manufacturer designed the cover so it rode close enough to any given pulley and it was made out of metal, you wouldn't even need one on any of the gears. Plastic is by far easier/cheaper to make then metal, so I completely understand why most wouldn't go this route.

NOTE: I've never considered it called a "walk blocker" before, but I guess the term works so no issues from me.

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  • Thanks a million Paulster, you gave me so much food for thought with your answer! In your 3rd point you said they use walk blockers to prevent eating through the plastic cover by the belt (if it walks off for any possible reason). However, I have seen so many cars with plastic covers and no walk blockers. I think cover is not important even if it is damaged by the belt. If the belt walks off the pulley, the engine will fail badly and in such case, no one thinks about the cover. I think this part needs more explanation. Jul 2 at 14:00
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    @NarimanAsgharian - I think you're not looking at #3 holistically ... I'm saying the walk blocker is there because of a plastic cover. The plastic cover in and of itself cannot contain the belt on its own. A metal cover could if tolerances were made close enough. Jul 2 at 14:05
  • Do you mean a metal cover with close tolerances (very close to the belt and pulley) can perform the same function as the walk blocker on the pulley? Jul 2 at 14:08
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    Not necessarily ... as long as everything is aligned, the belt won't walk (as @SolarMike was saying). Secondarily, if there's enough slack for the belt to walk, it would most likely skip a tooth before it'd walk off the gears (as long as the gears are aligned correctly). Proper maintenance will prevent this from happening (changing the belt at the correct interval). Jul 2 at 14:26
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    @NarimanAsgharian It's a fix for a problem that should have been designed out and never occur anyway. Back in the day, the critical "belt drives" on the engine were metal chains and sprockets which can't "walk" at all and lasted for the life of the engine. Then somebody had the half-baked "cost saving" idea of one long "serpentine belt" made from cheap materials that wore out in 30,000 miles, and then saved even more money with cheap plastic pulleys which also wore out in 30,000 miles. And when you have a system that is designed to fail, you need more features to mitigate the consequences.
    – alephzero
    Jul 2 at 14:54
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Because it can, or does, make fitting belts more difficult.

A walk blocker won't affect tension.

And belts only tend to come off the pulleys for two reasons (imhe) a bearing has failed and the pulley sits at an angle or the belt is delaminating.

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  • A walk blocker won't affect tension but it can prevent under-tensioned or over-tensioned belts from walking off the pulleys. So, according to what you said, they avoid walk blockers because they make it more difficult to install or remove a belt. It makes sense to me. Let's see what others say. Jul 2 at 10:31

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