# Belts keep coming off, or slipping



## Bob Driver (Nov 1, 2017)

I get a lot of machines in the shop where the complaint is the belts keep coming off, the belts keep slipping, or the belts roll over in the pulley. You have to bear in mind that not only do the belts wear, so do the pulleys. Any machine over 10 years old could have worn pulleys. They are called "V-belts" for a reason, the V-shape of the sheave groove grips the side of the belt. If the V-groove is worn to the point where it is now "U-shaped", either the belt will slip, or it will come off when a shock load is applied (blades engaged) because the belt will try to roll over in the pulley groove. If this keeps happening to your machine, you need to take a close look at the profile of your V-belt as it rides in the pulleys. If the top of the belt rides more than 1/32" BELOW the two edges of the sheave groove, the pulley is worn out. They actually make a set of gauges to check for pulley wear (Browning #2695179 Groove Gauge). The simplest way to check for a worn pulley is to use those gauges, otherwise you need to install a new belt and eyeball how a fresh belt rides in the sheave groove. I've had an older SCAG, and Exmark, both with HD cast iron pulleys in the shop recently. They were both leaving "stringers" in the cut of the center blade. Both customers had installed new blades to no avail. In both cases, the deck belt was riding a 1/4" below the edge of the sheave groove on the center pulley, the belt was slipping, and the center blade tip speed was off. Here's a link that explains the issue better in detail.
Demystifying the V-Belt Sheave Groove Wear - MROSupply.com

If you're getting "stringers" or a weird cut and want to check/compare spindle pulley speeds, to see if one is rotating at a different speed from the other(s), I use a digital laser tachometer. Get the spindle pulleys where you can point the laser at them and put them under a load (mowing)..... They should be turning at roughly the same RPM +/- 3%. The difference between "loaded" and "unloaded" spindle speeds is a really good way to tell how much a deck belt is slipping. A worn pulley won't read much of an RPM difference "unloaded", but under a load, it sticks out like a sore thumb
https://www.amazon.com/AGPtek®-Professional-Digital-Tachometer-Contact/dp/B004Q8L894/ref=pd_sbs_1?pd_rd_w=XmKMm&pf_rd_p=c52600a3-624a-4791-b4c4-3b112e19fbbc&pf_rd_r=CK0A05J3986586EJVRV4&pd_rd_r=18d2fb09-7bf0-4a43-b71c-59d28f37278b&pd_rd_wg=ijn3L&pd_rd_i=B004Q8L894&psc=1


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## BigT (Sep 15, 2014)

Excellent explanation Bob. Thanx!!


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## pogobill (Jan 31, 2012)

Yes, thank you Bob!


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