Open Belt Drive Systems

It depends on direction of rotation

Presuming the drive is horizontal as best I can derive from your description, the tight side would be the load (pulling) side. The no-load side will naturally be looser. A drive that uses a belt whose weight causes a noticeable catenary tension would still have the most tension on the load side. The belt will try its best to remain a round ring regardless of the tension applied, and will be "rounder" on the no-load side, hence "looser". A pulled string is straighter and tighter than a pushed string.

My sources are my own experience with the application of tensioning devices when deemed necessary due to centre distance restrictions of a particular drive. In many cases this lesser tension is of little consequence, especially if the centre distance is easily adjustable.

Ing. Robert Forbus

I am in full agreement with posts 1 and 2.

However, where the applied and/or resistive torques are rapidly varying, as might be the case for instance with a one cylinder engine driving a single cylinder compressor or pump, the belt will vibrate between the pulleys and it could appear that the tension (load) side of the belt was "looser". However this would not really be the case as would be demonstrated by measuring the average tension on both sides assuming you could do so. But it gets a little complicated because the very vibration described would serve to tighten the opposite side of the belt somewhat, and that would serve to accentuate the false impression. Depending on the particular system dynamics, it could also occur on something as common as a belt driven compressor and electric motor combination. All that is required for this to occur is that one or both of the drive or driven pulleys vary in torque applied to the belt at a frequency which causes the tension side of the belt to achieve resonant or harmonic nodes and it will appear looser, especially when only one node forms near the center of that side of the belt. The slacker side of the belt will not so vibrate, (any more than you could play a loose guitar string) so it could then easily appear tighter.

Likewise, again depending on the particualr system dynamics, and relative tensions the looser side of the belt could vibrate, making it seem even looser, and this is usually more common.

If I learned one thing in physics it is to be wary of "appearances".

I have no sources to cite specifically (Acoustics 101?) other than my own observations and insight, and as to justification - well this post is left justified, "formatically " speaking.

Perhaps your application has such a varying torque, and a debate has ensued over what you are "seeing".

Case 1: Top tight, bottom sag. This means 'less angle of contact' for the belt. This leads to 'more slip'.

Case 2: Top sag, bottom tight. This means 'more angle of contact' for the belt. This leads to 'more grip'.

A nice picture representing an oversimplification of a belt and pulley drive system with exaggeration of an improperly tensioned belt. It seeks to demonstrate that a belt laying loosely over a portion of a pulley contributes to the transmission of power: It does not! (Centripetal force and the linear momentum of a belt curving around a pulley explains part of this.)

With a properly designed system for the speeds and force (power) involved, and a properly tensioned belt(s), the drive direction does not matter. This has to do with how the power is transmitted between a properly sized pulley and belt and involves a number of factors that have little or nothing to do with the original question posted.

If however we are dealing with a poorly designed system, misapplication, or loose belt, it might, but all that says is: Do it right.

And that took a lot less than a thousand words!

Meanwhile, I'm still uncertain as to the exact purpose of the original question.

Umm... I think he meant to say 'which side OUGHT to be loose' since it seems obvious that the loaded side would be tighter. The post with the picture seems to answer that, although I don't know if it's the right answer. I think I agree with you you about the increased angle being defeated by centrifugal force on the driving pulley, but I think that the driven pulley would benefit from the droop although not much. If the belt was that loose I think the whole thing would slip like h3ll.

Gordie.