The Mechanic’s View of Control: Belt Drives
Rubber belts have lived under the hood of nearly every car since they first took to the streets, but they also transmit power through all kinds of industrial machine drives as well.
Mechanical power transmission is an important topic for all automation engineers. Even if the task is simply to design an electrical motor control system, you still must recognize how the motor will be delivering power to the end devices. Most motors do not attach directly to a load; nearly all machines have some sort of speed reducer, clutch, or distribution component to deliver the power to its final point of application and at the right speed with the proper torque.
Belt drives are among the oldest components in drive systems, and they existed long before electrical drive units achieved precise speed control. They can be an inexpensive way to transmit power, but there are a wide variety of belt geometries and technologies, just as there are with gear drive systems.
Figure 1. Motor driving pulley with a reduction in RPM but an increase in torque. Image used courtesy of Canva
Why Use a Belt Instead of a Gear or Chain?
A primary strength of belts is that they are easier to install than gears and chains, and they are a lot less expensive. They also have the added ability to provide some amount of slip, meaning that if the tensioning force or rotational speed is lowered, they will not transmit power, providing a very convenient way to disconnect various components or build in a ‘clutch’ if the load is too great.
Think of the way your lawn mower works: the pull of a lever simply tightens the tension on a belt that engages the blades and starts them spinning. Imagine how costly this task would become if a geared transmission or second engine were required!
Depending on the application, a few different kinds of rubber belts are employed, each with a certain set of strengths that make them an excellent choice for various applications.
There are a few disadvantages to belts. They don’t work well for low-speed systems. When we say ‘speed’ here, we must differentiate speed and RPM, even for rotating pulleys. The diameter of the drive pulleys can easily double or triple the RPM of the shafts, even though the belt is technically moving at the same linear speed at all points. So to understand the motion from the perspective of the belt, we must think in terms of the belt’s linear velocity in inches/min, ft/min, or m/s. Likewise, we may examine a sizing chart for V-belt pulleys (called sheaves) and see that they are sized in inches as long as the motor is rotating at a standard 1750 RPM.
Types of Belt Drives
To capture some of the benefits of flexible belt properties, many designs have been created over the decades that enhance strength, resilience to elements, and drive properties for all sorts of precision and variable speed systems.
Figure 2. Complex assembly lines of machines, like in this textile factory, can be driven by a central drive belt system. Image used courtesy of Canva
Certainly, flat belts are among the oldest representatives of the power transmission world. Even before rubber and synthetic materials appeared, these designs used leather strips to provide the necessary friction and flexibility to drive machines. The rare photos and drawings of industrial revolution-era factories and into the early 20th century show large arrays of machines driven from a single steam-powered shaft, supplying the rotational energy to a room full of large machines.
The flat belt looks exactly how it sounds: a flat, narrow strip of reinforced rubber (modern) tensioned between two round, similarly flat pulleys. Usually, the pulleys would have up-turned edges or a very slight crown profile, used to ensure that the belt does not slide off to the side if the tracking is uneven since the flat profile provides no friction to protect against side-to-side sliding.
These flat belts are easy to install and flexible, as well as very durable at high speeds. With the flat profile, they do not have as much tension or compression when turned against the pulley as compared to other profiles and will not be subject to cracking as early in life. They can commonly be found in applications where the pulleys are separated by a long distance, as the driving pulley ‘pulls’ (which, side note, the word pulley is unrelated to ‘pull’ but comes from the Greek polos, meaning rotation around an axis) against the tensioned side of the belt, and even if the belt is a bit loose, due to wear and temperature effects, it will still provide power.
Since they have no added friction as other belts with more surface area, they can be difficult to start under load or maintain rotation under heavy loads; both scenarios should be avoided.
The V belt is a common profile for older vehicle engines and is found under nearly all riding lawn mowers. This kind of belt is great for applications that are constantly connected and removed from the drive power. The taller profile of the belt provides more rigidity, so if even a small amount of external tensioning is removed, the belt will no longer contact the drive surfaces, and power is readily removed while the belt remains inside the pulley groove.
Figure 3. Typical application of a rubber V belt. This tensioner pulley rides on the back side of the belt and does not have the normal v-shape of the drive and driven pulleys.
This rigidity for repetitive de-coupling is the reason they are so common on lawn equipment, but it’s the very same reason they are not often found on modern vehicles, replaced by the far more common multi-groove, or ‘serpentine’ belt. You won’t find yourself engaging and disengaging a fan belt except at the beginning and end of its life!
Multi-groove V Belt
These multi-groove belts share many of the same properties as normal V belts, but instead of a single v-shaped profile, there are several small ones. This adds surface area for friction, but it reduces the height of the profile, looking far wider and flatter from a cross-section view. The reduced height means less tension and compression strain and longer life.
Figure 4. Belt and pulley for a multi-groove V belt.
One disadvantage of these belts is that, when tension is removed, they can be easily slipped off their grooved drive pulleys. This makes them ideal for applications where they are unlikely to ever be removed, except during replacement. They stay seated in the grooved pulleys exceptionally well with no risk of sliding sideways, but when even a slight amount of tension is removed, they can be slipped off and replaced.
Like the flat and V-belts, these grooved belts work best under high speed with relatively light loads. For very heavy loads, we often rely on the toothed belt, or we step up to metal chain drives.
Toothed Belt, or Timing Belt
Toothed belts are the rubber equivalent of chain drives. They are able to mesh with sprockets, having rounded teeth that provide traction and very precise turning force.
These belts also have the name timing belt because of their application in vehicle engines, coordinating timing of valves with the position of a crankshaft. In industrial applications, the purpose is less about coordinating timing, and more about small positional changes that have no room for error. For this reason, they are common in CNC machine axes when small degree changes from the drive motor to the carriage allow no room for slipping and often travel at low speeds under constant starts and stops. Those are normally the worst conditions for a belt, but the toothed profile is great in such situations.
Figure 5. Toothed belt with sprockets and a spring tension system from an older CNC machine.
Drive Belts in Industry
Linking motors to machines is a process with many variables to consider, which is why the field of engineering provides so many options. Flexible belts are one of the simplest and most reliable systems as long as they are used with respect to their limits and capabilities.