Do rubber drive belts *significantly* stretch if stalled?

[skotl]

My home-made-ish frankensander stalled the other day, and now the belt is so loose I think it's probably 25-33% longer than it was

Before pic:

Since that pic was taken, I added a swing arm with a bearing to take up the (what was) slight slack, but now the belt just flaps around even when the swing arm is at it's greatest extent.

So I guess the answer to my post's title is "yes" but just checking that belts really do stretch that much if they get stalled?

The belt does seem quite weedy so I guess I need to get a thicker / more substantial replacement.

Edit: original thread with close-up pics of the belt: https://www.mig-welding.co.uk/forum/threads/how-do-i-figure-out-timing-pulley-specs.139105

 

[Kram]

I would say no they shouldnt stretch any significant amount as they would normally have kevlar or similar, fibre reinforcement. Your belt does look very thin and likely under sized for the job! What power and speed is the motor?

I'd imagine either the motor or spindle pully, or both, have slipped on their bolt mountings - also check thst the belt is running square to both. I have had my lathe motor rotate and the belt slacken off like that.

As a cheapy upgrade I'd likely suggest an A or B section V belt, ( XPA/XPB belt ), but only becuase they are very simple to specify.
Others on the forum have suggested poly V belts before, technically better, as used on car drive belts but seems more complicated.

 

[m_c]

A common reinforced belt shouldn't stretch, but it could be you have one that's not reinforced, which will stretch.

 

[skotl]

Cheers - will do a more in-depth investigation tomorrow. It could be a cheap belt (Aldi original).

 

[frank horton]

If the cords let go on a belt it will just fall apart...
most trouble with "belt stretch" is old worn out pully's that rapidly wear the belt away
flat belts need to be a little tighter unless they are 20-30m long....look at old steam powered stuff...for them the tension is the weight of the belt....
in comparison, transmission of HP between a flat belt n a V belt, the flat belt needs to be quite wide n tight in comp....
cant think of any reason in machines where I use a flat belt unless it was toothed or needed emergency loss of drive....
but then there are better ways to stop the power than a skidding belt....
ur sander I would have used twin V belts for long service life...not an expensive option anymore....

 

[8ob]

Pulley's look way too small for the job, that’s very little belt to pulley contact for what could be a high torque application, that alone would cook a belt.

Bob

 

[bricol]

All the belt companies, Gates for example, have info on how to design a drive, select the belt, pulleys, the required tension in the belt etc.

Belts do stretch – I had a new design a few years ago, the kevlar reinforced belt was over a grand – could not work out why we were having issues. Fitted encoders to each shaft and logged positions. Working back from the angular displacement of one shaft from the other, we were getting 45mm elastic stretch in the belt.

Until we proved it, I would not have believed it.

 

[Migmac]

I have two old belts here and two new ones coming. New ones are identical to the old ones, I’ll measure and see the difference between old and new. I think there will be some stretch as I had run out of adjustment on the machines

 

[Morrisman]

When I lived in the Philippines local ‘mechanics’ used old fan belts for engine lifting straps. I was rather amazed, but they obviously knew they were fairly robust.

 

[tom2207]

probably more wear from the small pulleys

 

[skotl]

So I did some more investigation and am even more confused.

First, the belt: it doesn't feel stretched, and can't be stretched by hand. Placing it against a ruler I count 20 teeth in 10cm which seems bang on the 5mm pitch for the belt.
Next, the pulleys: one is brand new, both look unscathed.
Finally, the motor itself and the sander - both are rigidly bolted to the table and neither could have moved.

So I'm utterly bemused. I'm going to add a second tension wheel and/or move the motor back a cm or so. My best guess is that I imagined it being taught in the first place and have made an ass of mounting the motor.

The belt in question: