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Old 03-24-2019, 12:24 PM   #1
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Belt tension

Just curious how others of you tension your engine belts.
Too tight causes several issues including costly ones like water pump failure.
Too loose and they slip so the pump/alternator work suffers.

I think there must be a wide range of tolerability/forgiveness to this or there'd be more problems.

Just tried using a Krikit gauge to quantitate how much pressure I was applying but was surprised to find how much pry bar pressure is needed to get up to 50lbs - far more than I'd been stopping at when using my thumb as the instrument.
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Old 03-24-2019, 12:33 PM   #2
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On my SP225 Lehmans, I have 2 belts and just stock alternators so I donít need much tension. Brian at ADC said leave them fairly loose so I do.
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Old 03-24-2019, 12:58 PM   #3
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I do it by feel but I adjust and hold the tension via a turnbuckle. The turnbuckle ends are called an aircraft or compression fork or jaw to jaw. You have to watch the cooling fin clearance on the alternator against the turnbuckle. Ted on OC Diver has a new school set up on his refit which is definitely worth a look. Iíve lost one belt in 20k plus miles. Each alternator is 110 amp an externally regulated.
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Old 03-24-2019, 01:51 PM   #4
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I'm running a Volvo Penta 200 hp diesel.
Shop manual says max 10mm or 3/8 inch deflection at mid point between 2 pulleys. This seems about right to me manual or no manual.

BTW I have stayed with the VP OEM belts, a little more expensive but last much longer. The generic belts do not wear well and stretch more.
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Old 03-24-2019, 03:55 PM   #5
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Normally the correct tension for a v belt is 1/64 inch deflection per inch of span length.
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Old 03-24-2019, 04:22 PM   #6
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I generally go 1 belt width deflection. Also watch the belt when running. If it's not running straight. It's not tight enough.
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Old 03-24-2019, 06:27 PM   #7
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Quote:
Originally Posted by ORIF View Post
Just curious how others of you tension your engine belts.
Too tight causes several issues including costly ones like water pump failure.
Too loose and they slip so the pump/alternator work suffers.

I think there must be a wide range of tolerability/forgiveness to this or there'd be more problems.

Just tried using a Krikit gauge to quantitate how much pressure I was applying but was surprised to find how much pry bar pressure is needed to get up to 50lbs - far more than I'd been stopping at when using my thumb as the instrument.
Well, you are using the right tool, so you are light years ahead of anyone recommending thumb rules.

This is a V-belt, correct?

Sheaves have decent alignment? Straight edge or string is fine for this application.

50lbs is indicative of a small belt. Do you have the gauge in the centre of the span? Are you pushing perpendicular (90) to the belt? Should just be with your finger and only in the little finger hole. Takes a few tries to feel it click and stop pushing.

I've used all sorts of belt tensioners and it takes a bit of experience to get repeatable results with any of them.

I'm assuming that the pry bar is to move the alternator to set the sheave span. If you do the math you'll see that a the more modern adjusting screw on the alternator provides a heck of a lot of force with a simple turn of the wrench. So the pry bar feels like a lot, but it's just inefficient. Keep in mind that the tension is on both legs of the belt.

As far as tolerability or forgiveness, remember that our boats only get a few hundred hours on them a year (if we are lucky).

So really you can have it set incorrectly for years without ill effect, but it still isn't right.

If you are slipping under heavy charging load, well that's different. You'll glaze up the sheaves quickly.

I work with industrial equipment, which is generally 8,760 hours per year interval. Downtime costs money.

My experience is that most V-belts are found under tensioned as they are not checked often enough as they wear.
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Old 03-25-2019, 07:35 AM   #8
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I do it by feel but I adjust and hold the tension via a turnbuckle. The turnbuckle ends are called an aircraft or compression fork or jaw to jaw. You have to watch the cooling fin clearance on the alternator against the turnbuckle. Ted on OC Diver has a new school set up on his refit which is definitely worth a look. Iíve lost one belt in 20k plus miles. Each alternator is 110 amp an externally regulated.
Here is the link to the belt tensioning setup I built for my

Second Alternator

Ted
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Old 03-25-2019, 07:38 AM   #9
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I tighten just enough so that when I try to move the alt pulley by hand it does NOT slip.
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Old 03-26-2019, 11:20 PM   #10
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Thanks for all the input. Always good to hear different techniques for common tasks. I'm going to use a combination of the above described manual techniques and incorporate this Krikit gauge to see if I can get the hang of getting a more consistent tension set.
Thanks again
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Old 03-27-2019, 05:37 AM   #11
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The belt tension test is in the results.

Belt chunks or black dust on the engine , probably too tight or overloaded.

Keep replacing the alt because the front pulley bearing dies?

Its probably too loose and the heat from the belt slipping heats the pulley to melt the grease out of the bearing.
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Old 03-27-2019, 09:55 PM   #12
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Here is the link to the belt tensioning setup I built for my

Second Alternator

Ted
Beautiful, Ted.
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Old 03-27-2019, 10:42 PM   #13
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Beautiful, Ted.
Thanks!
My OCD (Obsessive Compulsive Disorder) hard at work.

Ted
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Old 03-28-2019, 10:47 AM   #14
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When we upgraded our alternator to 105A it came with a 2.8Ē x 10mm pulley. No matter how much tension I put on it, within 25-30 hours it would start slipping. After much research I found a 3.0Ē x 13mm pulley and bought a kriket tool. No more slipping with the tension at one half the recommended tension. Any more than that the belt is visably stretching. We will see on this trip if itís good enough. Also my tachometer is back to reading correct.
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Old 03-28-2019, 12:15 PM   #15
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High Wire,
What did you use as the source for the recommended tension?
I tried the tension calculator on the Gates site which involves plugging in 2 variables:
belt type (V belt in my case) and belt width.
That put the recommended tension at 120 lbs.

120 seemed high so I too tensioned mine to 50 or 60 which seems appropriate when applying all the other methods described in the above thread...
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Old 03-28-2019, 01:09 PM   #16
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I can’t remember where but the number for my alternator belt was 100. I never got there. 50 has been good enough to stop the slipping so far.
I used a breaker bar and stretched the belt for me was uber tight and kricket only said 75..
I didn’t want to break anything.
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Old 03-28-2019, 09:21 PM   #17
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I canít remember where but the number for my alternator belt was 100. I never got there. 50 has been good enough to stop the slipping so far.
I used a breaker bar and stretched the belt for me was uber tight and kricket only said 75..
I didnít want to break anything.
Exactly my experience. Thanks
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Old 04-04-2019, 01:56 PM   #18
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with a 20" center to center length, i have 3/8" to 1/2" slack. have a 6" piece of dowel with 3/8" and 1/2" marks on it. always remember, up to 100 amps, we can use one 1/2" belt, over 100 amps, i use 2 3/8 belts...for belts and hoses, i always use green stripe...last and last...clyde
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Old 04-04-2019, 05:27 PM   #19
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Here is the link to the belt tensioning setup I built for my

Second Alternator

Ted

That is a good looking system. I decided to look into large frame alternators, and came up with this from "Sinister Diesel"
Check it out....



QUOTE:
Sinister Diesel 320 Amp OEM alternators are trusted by many Diesel repair shops and stores to be the ďfix all solutionĒ for the GM Duramax 6.6L alternator failures which may cause electronics failures due to low voltage from the lack of alternator power that comes from the Ford factory. ....

I guess I won't look any further into these guys!
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Old 04-04-2019, 05:34 PM   #20
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Beautiful, Ted.
It's even more beautiful when seen in person! I had to wear sunglasses in OCD's ER.
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