Belt Basics

Belts are seldom used today in residential applications, but they are common in commercial applications. Understanding a few belt basics can make the commercial technician’s life much easier. The main type of belt used in HVACR is the V-belt, named for the V cross-section shape. The sides of the belt that form the V are what should be riding on the pulley, not the bottom of the belt. Belts are made of layers of cords encased inside rubber, similar to car tires. The cords provide strength.

Belt Identification
There are two predominant naming conventions for HVACR belts. One uses 3L, 4L, and 5L to designate belt widths of approximately 3/8”, 1/2”, and 5/8”. A number follows the with designation which gives the belt circumference in inches. So a 5L-440 is approximately 5/8” in width and 44 inches in circumference. The other common convention is A, B, and C for 1/2”, 5/8”, and 7/8” widths respectively.

Matched Belts
Large commercial applications may use multiple belts on the same drive. You should always ask for a matched set – not just two or three of the same size. This is because there can be small differences in length, which are of no consequence when using one belt. However, when using several on the same sheaves, they must be identical or some of the belts will be loose. A matched set is much closer to identical than just two belts with the same name size. This also means you should always replace all of the belts, even if only one of them needs replacing.

Installing a Belt
A common (and incorrect) way to install a belt is to roll it over the edge of the pulley. This saves time, but damages the cords in the belt – leading to shortened life. The correct way is to loosen the belt tensioner or motor mount so that the belt fits easily over the motor and fan pulleys. Then tension the belt using the belt tensioner or by adjusting the motor, depending on the design.

How Tight Should it Be
Many techs make the belt as tight as they possibly can get it. However, a belt only needs to be tight enough so that it does not slip at maximum torque. For fan belts, that is at start-up. Belt tension is measured by pressing in the middle of the belt while measuring both the force used and the amount of belt deflection. Browning recommends a deflection of 1/64 the belt span. The span is the center to center distance between the pulleys. So a belt with a 64" span should have a 1" deflection. The amount of force varies with the particular belt and span. Browning publishes a chart for their belts. A belt tension gauge is used to measure the deflection force. There are a couple of different ones out there. Again, Browning makes one.

Curing Belt Jitters
Many techs mistake misaligned belts as being loose. A belt that vibrates and jumps up and down usually indicates alignment issues. Remember that the shafts and pulleys must be aligned in three dimensions. The shafts should be parallel to each other. If you extend an imaginary line out from the shafts they should not intersect each other. You should be able to lay a straight edge across the pulley faces. If one pulley is farther forward than the other the belt will jump.

Worn Belts
Worn belts typically have shiny, glazed sides. If you see this, replace the belt. The hard, slick sides can’t grab the pulley wall, and so it slips even if the belt is correctly tensioned.

Belt Lubrication
Belts do NOT NEED to be lubricated. An old trick is to spray some WD-40 on a slipping and squealing belt to quiet it down. This basically just makes it slip quietly – just long enough for you to leave. It does not fix the problem. If anything, it makes the belt slip worse. It also does not last very long and leads to early belt failure.