Calculating Belt Tension

John Culp a member of the SEL had a need to generate power at his home and had a convenient diesel engine and wanted to belt it up to an alternator - he had a few belt slipping problems and too the rescue came the SEL... Thanks to Richard Allen and "Farmer".

Date: Mon, 1 Jan 2001 20:54:06 -0800 (PST)
From: Richard Allen 
Subject: Re: [Fwd: Re: Calculating Belt Tension]

Richard Allen wrote:

| John Culp wrote:
|
| || Well, I spoke too soon of my satisfaction with my tensioning
| || arrangement. The belt started slipping excessively under the mere
| || 1500 watt heater load and the turnbuckle came loose.
|

Tensioning a belt to the amount needed to prevent slippage and at the same time making sure it is not over-tightened can be easily calculated IF a few things are known first. Belt tension is real important to prevent belt wear from being too loose and to prevent premature bearing failure from being too tight. Another important aspect is to make sure the correct RPM is maintained, which is of utmost importance when running an alternator for household electricity generation. If the horse power or wattage being transmitted by the belt is known, and the Revolutions Per Minute of the pulleys are known, then the belt can be tensioned using a mechanic's torque wrench on the SMALLER of the two pulleys. The reason the smaller pulley is used is because it will almost always start to slip before the larger pulley will.

Two of the general mathematical formulas for engines and motors are as follows:
Horse Power = (Torque x Revolutions Per Minute)/5252

One Horse Power = 746 Watts = 0.746 kiloWatts

For these formulas Torque is expressed in pounds-feet (also called foot-pounds by some). The x symbol means "multiplied by", the / means "divided by".

We need to find Torque when tensioning a belt, so the one formula has to be re-expressed as:
Torque = (Horse Power x 5252)/Revolutions Per Minute

In John's case he was operating a 60 Hertz alternator at 1500 Watts then at 3100 Watts. His alternator has to run at exactly 3600 RPM to make 60 Hz AC electricity. Converting 3100 Watts into Horse Power is

3100/746 = 4.15549 Horse Power

We will round it off to 4.2 Horse Power because that is accurate enough for our purposes. In this case the alternator has the smaller of the two pulleys on it, so we use the RPM it runs at in the formula. Plugging in the Horse Power and Revolutions Per Minute into the Torque formula we have:
Torque = (4.2 x 5252)/3600

Torque = 22058.4/3600

Torque = 6.127 pounds-feet, rounded off to 6.1 for our use

So when the belt for the alternator is adjusted, the small pulley in the transmission will have 6.1 pounds-feet torque applied to it by hand with a mechanic's torque wrench, and the belt tensioned until the pulley no longer slips in the belt. It would be wise to go a little higher than 6.1 pounds-feet because there are other minor factors involved with slippage that have to be compensated for, too, like the belt getting hot. This kind of calculation is important where adjusting belts is not practical or safe while the equipment is running.

Hope this helps. Work and play safely,-------Richard Allen

Date: Sun, 31 Dec 2000 13:37:06 -0800
From: farmer 
Subject: Re: Calculating Belt Tension

This is old hat to most of you guys but I thought I would post it for those new to "belting up". I made a quick sketch and scanned it rather than get brain strain trying to think of the right words:

A. is simply two pulleys and a belt. B. shows how much more surface contact you can get especially on the smaller pulley. C. shows how you can actually lose contact area with the idler in the wrong spot. Of course the idler whether fixed or spring mounted should normally go on the slack side of the belt. An idler isn't a free ride and will ad slightly to the overall load but will allow you to increase traction significantly without having to increase belt tension to a near failure level.

--
farmer
Central Indiana USA

Small Farm Shop e-mail list at:
http://www.egroups.com/group/Smallfarmshop
Friendly new members welcome...


"Strive for-
Moderation-
Tolerance-
A mind open to new ideas...
Your life will be richer if not your purse."

This article is part of the Stationary Internal Combustion Engine Frequently Asked Questions (FAQ). This series is a combination of my views and knowledge and the views and knowledge of other people - most of them members of the Stationary Engine Mailing List (More info on ATIS). Those articles which were written by others are © Copyright to the author. Those articles written by myself are © Copyright to Paul Pavlinovich.

Got comments?

If you would like to comment on any part of the FAQ please do so by using the

Disclaimer:It should be noted that the information given in this document is considered to be good advice by the people who give it, however any legal liability lies strictly with the reader. The contributors are hobbiests not professionals.

Last modified Sunday, 20-Jul-2003 15:28:00 BST

This menu is included for the blind to use with speaking software that may not be able to cope with the java based popup menu at the top of the screen.

Home	Internal Combustion	Steam	Stirling Cycle	Railways	Miscellaneous
New Clubs Listing Museums Listing Site Reports Events New Year CrankUp Reports Announcements	Engines Restoration FAQ History Magneto Saw Mill Models Tractors On The Water SEL	Engines Restoration FAQ History Saw Mill Models Traction Road On The Water COALS	Engines History Operation Models	Mainline Narrow Gauge Tramways History Models	Site Map Search Models Identity Parade - Can you help? Registrars Listing by Type Manual Exchange Listing Guestbook Memories About Copyright Links

All documents and images on this site are
© Copyright 1995-2008 Paul Pavlinovich unless otherwise stated. You may not copy any documents or images from this site without explicit written permission except as allowed by Statutory License under The Copyright Act. More Information.

e-mail .
Please note that a clickable address is no longer provided due to spam harvesting. Just type the address from the image above into your favourite email client. Over time I will be replacing or removing all email addresses on this site with this sort of feature.
This site is best viewed with the free open source browser Firefox.