Belt tension

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Ian James

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I was looking at the Vincent pages and it seemed to me that all of the belt primaries have tight belts, one uses the gen, drive as a tensioner . Some have a double sided belt that must be designed somthing like a tire to de able to push with one side and be pulled on the other. My question is. Why does the Commando belt have to run slack? I have used a Norvile one (1/2 _3/4 twist on bottom run) and now have a CNW (lift bottom run to alternater gromett is it ? Cant remember ) and all that I have read has said that slightly loose is better than too tight. Finaly the camshaft belt in my car has jocky pully and is drum tight. Whats the storie please Cheers Ian
 
According to my Velo friend, the 1/2 twist is now replaced with an c/c measure (on some belt drives).
One possible reason could be that if adjusted cold, engine, plates and gearbox will grow and tighten the belt when warm. I know this happens to a Manx that the chain will tighten and break if too tight cold.
 
Mike40M said:
According to my Velo friend, the 1/2 twist is now replaced with an c/c measure (on some belt drives).
One possible reason could be that if adjusted cold, engine, plates and gearbox will grow and tighten the belt when warm. I know this happens to a Manx that the chain will tighten and break if too tight cold.
Click to expand...
I use the 1/2 twist - what do you mean by c/c measure - never heard of that?
Cheers
 
concours said:
"center to center" distance is used to calculate the amount of desired deflection.
Click to expand...
That would suggest that there is no stretch or wear during the life of the belt.
Maybe that's so?
I have not found that to be true.

Actually - if you get the C/C right and measure the belt twist, why can't that be the gauge?
 
robs ss said:
That would suggest that there is no stretch or wear during the life of the belt.
Maybe that's so?
I have not found that to be true.

Actually - if you get the C/C right and measure the belt twist, why can't that be the gauge?
Click to expand...
I didn't explain clearly enough (slugging coffee, launching for work).
"Center to center" is a (rough) value used to calculate how much deflection, based on the SPAN, (think a short chalk line versus a long one) is appropriate for the belt in THAT setup.
NOT to measure "center to center" when adjusting the tension.


The chart below is merely AN EXAMPLE of center to center distance for calculating belt deflection. NOT INTENDED to be applicable to Commandos, primary belt drives, or anything besides v-belts.
577A6404-9639-4530-986C-FA08C17A921B.jpeg
 
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"
In order to have a MEANINGFUL SPECIFICATION, it must read like: "3/16" deflection using 18 lbs. force."
To read/listen "half specifications" repeated around the campfire is painful.

I completely agree with your statement but most specs I have ever seen for chains/belts simply state some amount of deflection without any force spec.
 
Ian James said:
I was looking at the Vincent pages and it seemed to me that all of the belt primaries have tight belts, one uses the gen, drive as a tensioner . Some have a double sided belt that must be designed somthing like a tire to de able to push with one side and be pulled on the other. My question is. Why does the Commando belt have to run slack? I have used a Norvile one (1/2 _3/4 twist on bottom run) and now have a CNW (lift bottom run to alternater gromett is it ? Cant remember ) and all that I have read has said that slightly loose is better than too tight. Finaly the camshaft belt in my car has jocky pully and is drum tight. Whats the storie please Cheers Ian
Click to expand...
The Newby belt drive on my Vin runs with approximately the same slack as the RGM belt drive on the Norton.
The pulley centres are fixed on the Vincent, so Bob Newby sends out the correct length belt for the job. If memory serves the total deflection midway between the pulleys is about 3/4" . Both with RGM and Newby, there is no lift pressure given, just use hand pressure as you would when setting a drive chain to 3/4" total deflection. There is no setting on the Vin, but I also installed a Newby on the 650ss. The instruction sheet for that detailed adjustment for proper slack.

I don't see any of these belts ever stretching or changing in length as they are really a series of continuous steel cables encased in rubber.

That's why a cush drive is still needed, there is even less give in a belt drive than in a chain.

Glen
 
robs ss said:
That would suggest that there is no stretch or wear during the life of the belt.
Maybe that's so?
I have not found that to be true.

Actually - if you get the C/C right and measure the belt twist, why can't that be the gauge?
Click to expand...
I didn't explain clearly enough (slugging coffee, launching for work).
"Center to center" is a (rough) value used to calculate how much deflection (think a short chalk line versus a long one) is appropriate for the belt in THAT setup.
NOT to measure "center to center" when adjusting the tension
MexicoMike said:
"
In order to have a MEANINGFUL SPECIFICATION, it must read like: "3/16" deflection using 18 lbs. force."
To read/listen "half specifications" repeated around the campfire is painful.

I completely agree with your statement but most specs I have ever seen for chains/belts simply state some amount of deflection without any force spec.
Click to expand...
I agree, but think about the variation to that. It kinda assumes a seasoned mechanic knows how much force to use to twist or deflect.
For example, from a Lycoming diesel service manual: "cylinder head torque: one man on a 6' wrench" in that case, 1000 to 2000 foot pounds will work depending on the size of the gorilla.
Same thing with belts.

The Commando belt drive is a compromise, to be sure. That is, to set up the belt to a tension the BELT would last the longest and be most efficient, would be WAY TOO TIGHT for what the clutch, overhung on the mainshaft, supported with sleeve gear bushings can tolerate.
So really, it comes back to as tight as the rest of the stuff not designed to have a belt & sprockets on it will tolerate. So, "as snug as decent clutch (therefore gearbox operation) operation will accept. "
 
I was involved setting these up, starting in 1985, (industrial applications) and learned about the belts, sprockets and how to get the best performance. If someone wants to plug in the numbers, we can review those specified deflection values.

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Belt tension
 
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Rattling around in my brain:

The C-C distance between the engine sprocket and the clutch sprocket is not a fixed value when operating. Because of the co-axial input/output design of the transmission and varying states of wear, the reaction force of the final drive chain has to be accounted for. So any sort of "tensioning" has to take this into account - as you would with a chain primary.

So hence, the half-turn twist of the belt rule-of-thumb.

As always - if it ain't broke, don't break it!!

My $0.02...
 
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Interestingly, a common shop "adjustment" for a belt-driven cam on Ducati twins was to insert a standard #2 pencil between the belt and the cam gear then turn the engine over by hand. Cam belt tension was "correct" when there was just enough tension to jam the pencil where you could not turn the engine. If the pencil was "carried" around the gear with the belt, it was too loose! One could call that insanely unprofessional but I saw it done frequently, even on the race track! I reluctantly admit that's how I adjusted the belt on my '91 900SS. :eek:
 
MexicoMike said:
Interestingly, a common shop "adjustment" for a belt-driven cam on Ducati twins was to insert a standard #2 pencil between the belt and the cam gear then turn the engine over by hand. Cam belt tension was "correct" when there was just enough tension to jam the pencil where you could not turn the engine. If the pencil was "carried" around the gear with the belt, it was too loose! One could call that insanely unprofessional but I saw it done frequently, even on the race track! I reluctantly admit that's how I adjusted the belt on my '91 900SS. :eek:
Click to expand...
Agreed and understood. I align spindle rotation grid shift for the CAT40 ATC arm on a milling machine BY EYE. Because I've done it hundreds of times, using the special alignment fixture works, but is slower. I've done hundreds, and can get the same results by eye faster. ;)(Same principle as a military "peep' site on a rifle, our eye can do amazing things when trained)
Customer says: "WOW, I've never heard it change tools so smoothly!":cool:

BUT, a good solid specification/procedure helps those that haven't done it hundreds of times get a good result. (Geenhorn/different or new task, etc).
 
worntorn said:
The Newby belt drive on my Vin runs with approximately the same slack as the RGM belt drive on the Norton.
The pulley centres are fixed on the Vincent, so Bob Newby sends out the correct length belt for the job. If memory serves the total deflection midway between the pulleys is about 3/4" . Both with RGM and Newby, there is no lift pressure given, just use hand pressure as you would when setting a drive chain to 3/4" total deflection. There is no setting on the Vin, but I also installed a Newby on the 650ss. The instruction sheet for that detailed adjustment for proper slack.

I don't see any of these belts ever stretching or changing in length as they are really a series of continuous steel cables encased in rubber.

That's why a cush drive is still needed, there is even less give in a belt drive than in a chain.

Glen
Click to expand...
Great stuff. Please, don't tell all those guys that claim the belt "is easier on the transmission" .... they've beleived that so long, the earth would stop and rotate in reverse first. Kevlar/steel filament belts don't stretch. (except for the last few revolutions before they soil the sheets.)
 
Derek Wilson said:
Rattling around in my brain:

The C-C distance between the engine sprocket and the clutch sprocket is not a fixed value when operating. Because of the co-axial input/output design of the transmission and varying states of wear, the reaction force of the final drive chain has to be accounted for. So any sort of "tensioning" has to take this into account - as you would with a chain primary.

So hence, the half-turn twist of the belt rule-of-thumb.

As always - if it ain't broke, don't break it!!

My $0.02...
Click to expand...

OP: " My question is. Why does the Commando belt have to run slack?"



From above:
So really, it comes back to as tight as the rest of the stuff not designed to have a belt & sprockets on it will tolerate. So, "as snug as decent clutch (therefore gearbox operation) operation will accept. "
 
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