Idler pulley speed limit

[ftpiercecracker1]

Just like the title says. Does anybody know a reasonalbe rpm limit for idler pulleys?

 

Need to downsize a pulley and want to make sure I'm not pushing the limit.

 

Max crank rpm will be 3,500 on a 7in (dia) HB. Idler pulley maybe as small as 2.5 (dia) which at 3500rpm corresponds to 9,800 rpm.

[Minuit]

What kind of bearing is in the idler? Is this a normal stamped automotive idler pulley with a sealed roller bearing in it or is it something else? To my mind, the main considerations would be the bearing. If you can find out what bearing your pulley has you might be able to pull up a maximum speed rating for it. If not, there are a few formulas out there to calculate a speed given the size and type of bearing.

 

A stock '91 4.0 harmonic balancer has a 167mm pulley diameter. The idler has a 90mm outer diameter. That means at the 4.0's 5300 rpm redline the stock idler pulley is turning just short of 9800 rpm. Now the pulley manufacturer won't expect you to run at redline for any sustained amount of time, but the pulley is good for it at least in short bursts. I didn't go measure my truck to make sure these numbers were right, but if they are right 9800 rpm isn't a crazy number. If I were in your position, I'd just make sure to use a pulley with a good quality bearing in it and not worry about it. I think you'll be ok.

[ftpiercecracker1]

2 hours ago, Minuit said:

What kind of bearing is in the idler? Is this a normal stamped automotive idler pulley with a sealed roller bearing in it or is it something else? To my mind, the main considerations would be the bearing. If you can find out what bearing your pulley has you might be able to pull up a maximum speed rating for it. If not, there are a few formulas out there to calculate a speed given the size and type of bearing.

 

A stock '91 4.0 harmonic balancer has a 167mm pulley diameter. The idler has a 90mm outer diameter. That means at the 4.0's 5300 rpm redline the stock idler pulley is turning just short of 9800 rpm. Now the pulley manufacturer won't expect you to run at redline for any sustained amount of time, but the pulley is good for it at least in short bursts. I didn't go measure my truck to make sure these numbers were right, but if they are right 9800 rpm isn't a crazy number. If I were in your position, I'd just make sure to use a pulley with a good quality bearing in it and not worry about it. I think you'll be ok.

 

Current bearing is a NSK 6203DU2, 100% Chineseium. 

 

I want to be able to use what I can get from an auto parts store, but will spend a little more for a better bearing if available, again, only if I can get it from the same place. I.E. I'm not special ordering anything.

 

I agree 100% the bearing is the failure point.

 

 

My HB dia is 184mm/7.25in and the current pulley is 89mm/3.5in dia.

 

If I can find and safely reduce to a 76mm/3in or better a 63.5mm/2.5in that is what I need to do.

 

I will check on the bearing details and see what I find. 

[AZJeff]

5 hours ago, ftpiercecracker1 said:

 

Current bearing is a NSK 6203DU2, 100% Chineseium. 

 

I agree 100% the bearing is the failure point.

 

I will check on the bearing details and see what I find. 

Just because the bearing is made in PRC isn't automatically mean it's crap.  

 

NSK is a worldwide brand (just like Timken, Toyo, Fafnir, etc.) NSK probably has factories worldwide, and they all build to the same corporate standards.

 

The much-vaunted Timken front hub bearings for the MJ/XJ are made nowadays in the Czech Republic, and meet the same standards as the old US-made hubs.

[ftpiercecracker1]

2 hours ago, AZJeff said:

Just because the bearing is made in PRC isn't automatically mean it's crap.  

 

NSK is a worldwide brand (just like Timken, Toyo, Fafnir, etc.) NSK probably has factories worldwide, and they all build to the same corporate standards.

 

The much-vaunted Timken front hub bearings for the MJ/XJ are made nowadays in the Czech Republic, and meet the same standards as the old US-made hubs.

 

 

I never said it was crap,  but thanks for the reassurance anyway. 

 

Do you have any knowledge regarding the question at hand?

 

FPC

[Jeep Driver]

Years ago I built a belt driven pump drive for a tractor. I sourced a couple of bearings for the shaft from NAPA, I don't remember the brand, maybe NAPA. The bearings didn't last long, they were cheap. 

 

I then sourced SKF, I was told those bearing were good beyond 20,000rpm, never a problem after that. IIRC the SKF bearings were something like $40 each. 

 

 

 

SKF makes idler pulleys designed specifically for what you want to do............beyond 10,000rpm is not a problem. And yes, idler pulley bearings are specific because of the amount of tension from the belt. 

 

Pay for it once and won't worry about it. 

 

https://www.summitracing.com/parts/skf-acp38001?seid=srese1&cm_mmc=pla-google-_-shopping-_-srese1-_-skf-bearings&gclid=Cj0KCQiA-8PjBRCWARIsADc18TLdl8Q9xrKK8p040-d8tVQ_aS6EuReZIzS-mGRjAnmaCuQJHb7gye8aAqtuEALw_wcB

 

 

 

[cruiser54]

The bearing in that idler pulley is the same as an alternator bearing. 

[AZJeff]

If Cruiser54 is correct, the pulley diameter (and hence maximum speed) of the alternator will be about the same as what the OP is proposing.

 

Find what brand/size of bearing is used in the alternator, and you would  be good to go.