Correct V-belt tension?

[ernst]

As I wrote in another thread http://www.moyermarineforum.com/foru...t=11118&page=2 I just replaced my V-belt. It is the standard Gates belt that comes with the Indigo (mechanical) fresh water cooling system. I looked up the specs on the Gates web page, they say the belt tension should be in the range of 85-115 pounds (after running for a few minutes; for a brand new belt it is 130#). I set it to approx. 90 pounds.

Obviously, too low is bad and my understanding is that too high is worse (potential bearing damage). Are there recommendations by Universal or MMI for the correct belt tension?

thanks

[Administrator]

What does Indigo say?

Bill

[ernst]

What does Indigo say?

Bill

Excellent question!

I asked Indigo and got a reply from Tom, within a couple hours! He says he never measured the tension formally. His recommendation is that the belt should deflect by 1/4" when the belt is pressed by the thumb midway between the coolant pump and the alternator, with a force of 5 pounds.

So, I guess if nobody else has a better idea, I will have to try to get a feel how 5 pounds pressure feels on my thumb. Does nobody use a tension gauge??

[ndutton]

It's a two part question: What is the recommended tension and how do you measure it?

Recommended Tension
It depends. Required tension is different for different purposes. For example, an alternator will likely require less tension than a pump. Belt contact with a pulley is a big factor too. To get the same power from the same size pulley with a belt in contact 90° will take more tension than a belt in contact 180°. Both of these factors apply to your question because you have both a pump and an alternator on your engine and the Indigo mechanical pump has very little belt contact plus it reduces the alternator pulley contact. Neither Universal nor MMI* publish a recommended belt tension but if they did it would not apply with the Indigo system installed for the reasons mentioned. Because of that, Administrator Bill's comment/question about seeking out Indigo's recommendation for their system is spot on.

Once you have a tension spec, how to measure it
Belt deflection for a given side force has been the old school shade tree mechanic method for ages but its accuracy is affected significantly by the free belt length. We had a member with a flywheel PTO alternator drive pulley and a much longer belt. Applying the same ¼" @5 lb. side force deflection specification on his setup will require much more belt tension than a stock A4. Further, how do you apply a calibrated side force? That's not all. You mentioned a Gates belt specification of 85 - 115 lbs. How do you measure that? How does the deflection method translate to the Gates spec? Then we get into free belt length variations again.

Your mention of overdoing the tension and its effect on bearings is noted. We do not have a history of alternator bearing failure but you're correct, overdoing the tension will certainly affect the bearings. How much and over what time is on a sliding scale.

*Although MMI has never published a tension recommendation, they offer a calibrated belt tensioner product in their catalog with installation instructions that result in a predictable tension value. The measurement dynamic used in the development of the tensioner was torque applied to the alternator pulley before belt slippage. Using this type of measurement eliminated the variables of side force, pulley contact and free belt length. The specification used in the development was the alternator belt tension specification from Lycoming aircraft engines, 12 ft-lbs. of torque applied to the alternator pulley with no slippage. Note that this spec applies to alternator loads and may not be applicable to pump loads.

I hope this helps

[Marian Claire]

Could you use a fish scale? Hook it to the belt at the mid point and pull till it reads 5 lbs and see what the deflection is.
Dan
S/V Marian Claire

[ndutton]

Could you use a fish scale? Hook it to the belt at the mid point and pull till it reads 5 lbs and see what the deflection is.
Dan
S/V Marian Claire

If trying to apply the deflection specification, yes.

[ernst]

It's a two part question: What is the recommended tension and how do you measure it?

Recommended Tension
It depends. Required tension is different for different purposes. For example, an alternator will likely require less tension than a pump. Belt contact with a pulley is a big factor too. To get the same power from the same size pulley with a belt in contact 90° will take more tension than a belt in contact 180°. Both of these factors apply to your question because you have both a pump and an alternator on your engine and the Indigo mechanical pump has very little belt contact plus it reduces the alternator pulley contact. Neither Universal nor MMI* publish a recommended belt tension but if they did it would not apply with the Indigo system installed for the reasons mentioned. Because of that, Administrator Bill's comment/question about seeking out Indigo's recommendation for their system is spot on.

Yes, absolutely. And you make me appreciate that this question is more complex than I thought.

Once you have a tension spec, how to measure it
Belt deflection for a given side force has been the old school shade tree mechanic method for ages but its accuracy is affected significantly by the free belt length. We had a member with a flywheel PTO alternator drive pulley and a much longer belt. Applying the same ¼" @5 lb. side force deflection specification on his setup will require much more belt tension than a stock A4. Further, how do you apply a calibrated side force? That's not all. You mentioned a Gates belt specification of 85 - 115 lbs. How do you measure that? How does the deflection method translate to the Gates spec? Then we get into free belt length variations again.

The Gates belt specification is from their web page https://www.gates.com/us/en/resource...ion-calculator
for a standard 3/8" belt. I measured it using the Krikit gauge https://www.gatestechzone.com/en/too...tension-gauges. It is a tight fit but I was able to _just_ get it on the vertical part of the belt, between accessory drive and alternator.

Your mention of overdoing the tension and its effect on bearings is noted. We do not have a history of alternator bearing failure but you're correct, overdoing the tension will certainly affect the bearings. How much and over what time is on a sliding scale.

*Although MMI has never published a tension recommendation, they offer a calibrated belt tensioner product in their catalog with installation instructions that result in a predictable tension value. The measurement dynamic used in the development of the tensioner was torque applied to the alternator pulley before belt slippage. Using this type of measurement eliminated the variables of side force, pulley contact and free belt length. The specification used in the development was the alternator belt tension specification from Lycoming aircraft engines, 12 ft-lbs. of torque applied to the alternator pulley with no slippage. Note that this spec applies to alternator loads and may not be applicable to pump loads.

I did not know about this tensioner, I just found it in the catalog. I actually have a belt tensioner but it is just a curved arm, without the spring thing that I saw in the MMI catalog. I know I installed it but don't remember when and where I got it, it may have come with the Indigo FWC system? Tension is regulated by turning a bolt at the top of the arm that pushes the alternator mounting bolt down. It works very well but of course it does not tell me what the tension is.

I hope this helps

Yes, this is very helpful. I did not appreciate how complex this really is.

My preliminary conclusion is that it is not 100% clear what the optimal belt tension is in my specific situation. Not having any strong data, and not wanting to rely on my thumb (not enough experience), I think I am back at that belt manufacturer recommendation, of 85-115#. At least this is a pretty wide range, so hopefully if I am staying in the middle of it I should be not too far off an acceptable value.

[ernst]

Could you use a fish scale? Hook it to the belt at the mid point and pull till it reads 5 lbs and see what the deflection is.
Dan
S/V Marian Claire

Ah, didn't see that. Clever!