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.

Understanding Centrifugal Fan Motor Performance

Since air is what we work with it makes sense to insure that our students understand airflow and fan performance. Fan motor performance is one of the most often misunderstood aspects air conditioning systems. The amp draw on a centrifugal fan goes down as resistance to airflow is increased. For most people this seems counterintuitive. It is easy to picture the fan motor pushing harder to overcome the resistance and increasing in amp draw. However, this is exactly backwards. Centrifugal fans move air by throwing the air outwards through centrifugal force. The amount of air the fan is moving decreases as the resistance to airflow increases. If the fan blades are moving less air, they can actually spin easier because there is less air to sling. This causes the motor RPM to increase and the motor amp draw to decrease.

The most convincing way to teach this concept is to have students figure it out for themselves using a centrifugal blower. Have them operate a centrifugal blower in free air with no restriction and measure both the amp draw and the fan RPM. Note that most centrifugal blowers cannot operate in free air for an extended time without overheating, so try and keep the free air operating time to a minimum. Next have them block one side of the air intake with a piece of cardboard and recheck the amp draw and RPM. Typically the increase in RPM is immediately obvious, but measurements prove the point. Have them slide the cardboard to block the intake only half way while watching the amp draw. A few minutes of experimentation will convince the students that blocking the intake actually causes an increase in RPM and a decrease in the motor amp draw. Next have them partially block the fan outlet while checking the amp draw. Once again, the amp draw will decrease. Allow them a few minutes of play time to convince themselves. This experiment does more to explain centrifugal blower motor performance than a week’s worth of lectures.

Now that you have them hooked, refer them to Unit 56 Fans and Airflow inFundamentals of HVAC/R where they can read about the characteristics of different type of fans used in the industry and the basic principles of airflow. There the students can see examples of the different types of fans and read about their performance characteristics. Unit 56 Fans and Airflow wraps up with a discussion of the fan laws and using fan perfromance tables and curves. As always, examples show in detail how to apply each of these concepts.

Note that what I have been discussing assumes a "regular" PSC blower motor. ECM blower motors behave differently because they are programmed to adjust their output according to the resistance they are working against, but that is an entirely new discussion which I will save for another article.

Safety note: If you are not sure all of your students understand that it will hurt to put their hands into a moving fan blade, you should put a wire gaurd over the intake and exhaust to keep hands and fingers out. For a more polished trainer build sliding sheet metal baffles for both the intake and exhaust and mount the blower to a stand.

To see all my posts be sure to vist hvacrfundamentals.blogspot.com