How important is pulley diameter?

[Chris142]

My Jeep uses a spring loaded belt tensioner like most cars do today. The pulley has a bearing in it and the plastic pulley is molded around the bearing so I can't replace the bearing.

The pulley is 70mm tall and 26mm wide where the belt goes.

The only pully available @ 4:30 tonight was a 76mm x 26mm pulley. So I put it on to get me home.

It seems to work fine but can the diameter change the belt tension somehow? There is still plenty of adjustment left in the spring loaded tensioner.

 

[Colt45ws]

Id say its fine.

 

[labman]

I agree. What is it, the alternator? Whatever, it will be running about 10% slower. Most of your accessories cycle on and off so that means it will be on about 10% more of the time. The belt will also be very slightly tighter, likely about the amount the belt has stretched since new.

 

[Chris142]

It's on the spring loaded belt tensioner. Had to get a new belt today while all this was going on. Thing froze and burned the belt off.

 

[danster]

No, this is the tensioner/idler pulley for a single serpentine belt that drives all of the accessories. Because the belt is driven by the (unchanged) crank pulley all of the accessories will turn at the same speed as before.

The slightly larger tensioner/idle pulley will "push" a tad more on the belt. You will likely just be restoring the tension to something closer to the original spec if your motor is a few years old. The extra width won't matter as long as the pulley fit.

 

[Chris142]

It's not wider it's taller. I was wondering how that effected the belt tension if any. Jeep has 98K on it now.

 

[Chris142]

I originally ordered the pulley that it calls for. It was the correct diameter but too wide to fit between the tensioner arm and the timing cover. So I had to resort to a pulley with a larger diameter.

 

[Kruse]

Originally Posted By: Chris142

The pulley is 70mm tall and 26mm wide where the belt goes.

The only pully available @ 4:30 tonight was a 76mm x 26mm pulley. So I put it on to get me home.


The increased diameter will probably make up for the tension that the spring has lost over the years.
Watch it for a few days or a week and if it works fine, run with it.
BTW, there is some latitude there because belt length varies from manufacturer to manufacturer.

 

[Steve S]

The pully is an idler pulley it doesn't drive anything.

 

[wantin150]

Tension is tension right? If you reset the tensioner pulley by cycling it, I would expect any issues.

 

[onion]

I wouldn't worry about it.

 

[dwendt44]

The increased diameter will probably make up for the tension that the spring has lost over the years.
Watch it for a few days or a week and if it works fine, run with it.
BTW, there is some latitude there because belt length varies from manufacturer to manufacturer.


+1

 

[Ronn]

Originally Posted By: Chris142
My Jeep uses a spring loaded belt tensioner like most cars do today. The pulley has a bearing in it and the plastic pulley is molded around the bearing so I can't replace the bearing.

The pulley is 70mm tall and 26mm wide where the belt goes.

The only pully available @ 4:30 tonight was a 76mm x 26mm pulley. So I put it on to get me home.

It seems to work fine but can the diameter change the belt tension somehow? There is still plenty of adjustment left in the spring loaded tensioner.


If the tensioner spring (or hydrualics) is set to maintain the *stock* tension range, then it doesn't matter. A larger diam wheel will create more *pretension*, but the tensioner mechanism is preset for a given tension, so it will net out. If it's a spring, then you're basically *preloading* the belt with a bigger wheel and the spring won't move as much to reach the desired tension. If the wheel was too big, you would have maxed out the tensioner and it wouldn't move any further. If there is *room* for more movement you're fine, since you didn't over tension it.

 

[eljefino]

What's the smallest difference between belt sizes? An inch? Half an inch? This is static, comparatively. My geometry stinks but it should be 3mm times pi or around 10mm of extra belt length getting gobbled up. Back when belts were manually tensioned people used to reef on them and stuff mostly survived.

I'm chagrined by all the answers... to questions you didn't ask, by folks who don't read stuff through.

 

[mechtech2]

It will work fine, as long as there is clearance and the spring in the tensioner works.

It may also have the benefit of lasting longer, due to slower RPMs for any situation, and longer bearing life.
Also, more plastic will have less surface wear than less plastic.

 

[wapacz]

Originally Posted By: eljefino
What's the smallest difference between belt sizes? An inch? Half an inch? This is static, comparatively. My geometry stinks but it should be 3mm times pi or around 10mm of extra belt length getting gobbled up. Back when belts were manually tensioned people used to reef on them and stuff mostly survived.

I'm chagrined by all the answers... to questions you didn't ask, by folks who don't read stuff through.


It would also depends on how much of the belt made contact with the pulley.

 

[another Todd]

It will be fine. Leave it.

 

[Gary Allan]

Originally Posted By: eljefino
What's the smallest difference between belt sizes? An inch? Half an inch? This is static, comparatively. My geometry stinks but it should be 3mm times pi or around 10mm of extra belt length getting gobbled up. Back when belts were manually tensioned people used to reef on them and stuff mostly survived.

I'm chagrined by all the answers... to questions you didn't ask, by folks who don't read stuff through.


I'm trying to wrap around what you're saying here (it's early, but I don't know if that matters in my case).

Form the belt in a circle (for imagery) ..put the regular set of pulley's on the interior ..remove the stock tensioner ..replace it with one 2.5X as big ..if it fits in the circle ..the center of the idler moves inboard to the center of the circle formed by the belt. The belt, other than tension upon it isn't altered whatsoever.

I don't see what you said and what I said as being the same thing. As long as he can put the belt on ..the difference is tension.

 

[Rabbler]

Radius difference is only 3mm. You'll have slightly more tension than you had before but I doubt it will cause any problems. As mentioned above your tensioner spring has likely fatigued a bit over the years this probably helps make up the difference.