Effect of belt tension on bearing loading:

[electricpete]

Effect of belt tension on bearing loading:

Let's say I have 75hp being transmitted through a vee-belt to a 1ft diameter sheave rotating 1800 rpm.

The torque is T = 5250 * 75 / 1800 ~ 225 ft-lbf (round numbers).

The force associated with that torque (in absence of any adjustment factor) would be 225 ft-lbf / (0.5 ft radius) ~ 450 lbf.

There is a 1.5 belt factor. So the bearing loading is approx 1.5*450 ~ 725 lbf

How does the belt tension enter into this relationship? Higher tension gives higher belt factor?
Does increasing belt tension by 150% increase bearing loading by 150% ?

Is there a way to estimate bearing loading due to belts alone in static condition? Let's say belts are tensioned to produce 8-12 pounds at 7/8" deflection in a 72" span.


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[dvd]

I've been down the same road asking about static tension for polychain gt2 belts. Gates wouldn't or couldn't address this issue. I think that part of the problem is that the static load due to belt tension disappears when the belt undergoes a dynamic load. The belt is static tensioned so that it doesn't become too loose under dynamic load due to belt stretch. The whole point of following the manufacturer's recommendation for tensioning is so that the belt can stretch the proper amount under load and maintain some tension in the slack side.

 

[electricpete]

I have other reasons for wanting to know the static tension.

We have a twice shaft speed vibration occuring. Wondering if it is a result of the static belt tension pull acting on a shaft which has a very very long keyway and may have stiffness variation at twice per revolution. A test is proposed to rotate the shaft under static conditions and use dial indicator (recording every 45 degrees) to see if shaft or sheave displays a twice per revolution movement during that static runout check. To get a swag of the validity, I would like to understand the relative force during operation and during static test.

Here is one proposed calculation:
10 pounds applied to produce 7/8" deflection in a 72" span.
Let's say round numbers 1" deflection in a 72" span.
The angle is theta = (1/72).

The tension of the belt during the tensioning test is
10 pounds perpendicular pull
* (72/1) (trigonometric factor)
/ 2 (two segments of the same belt each opposing the 10 pounds perpendicular pull)
____
360 pounds belt tension during tensioinng test.

Does this roughly stay the same oncethe perpendicular pull is released (I would think it might go down).
If it did, then I would estimate load at 360 per belt half = 720 lbf.


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[dvd]

It's hard to get answer out of people who can't read, but your local power transmission supplier might have access to one of these gizmos.

http://www.gates.com/brochure.cfm?brochure=2787&location_id=5013

 

[dvd]

I meant me as the one who can't read, or write, very well today.

 

[desertfox]

Hi electricpete

The static load on the bearing would be the resultant force
of belt tension on both tight and slack side vectorially added.
During rotation however the tensions are reduced due to the belt mass and velocity ie;- centrifugal force.
Therefore your bearing should see a reduction in load during
operation.

regards

Desertfox

 

[electricpete]

Thanks guys.

dvd - That is interesting stuff about sonic tensioning. I also found this link which gives formula for static tension in terms of some table lookups... not sure exactly how they got there... and only applies to predator belts...I'm interestd in analysing Gates Super HC V-belts (5V-140-0).

http://www.gates.com/downloads/download_common.cfm?file=PredatorSinglesPANote.pdf&folder=brochure

desertfox - I would suspect the slack side tension is lower during operation... but the tight side is transmitting the load torque. I haven't worked through my forumula's yet, but I suspect that would end up being higher than the static tension.

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[MikeHalloran]

You also need to evaluate the degree to which belt tension bends the shaft. Of course it gets worse as the shaft gets smaller, as the cantilever distance gets larger, and as the span between bearings gets smaller.



Mike Halloran
Pembroke Pines, FL, USA