One method of driving your mobile compressor is to run it off the engine by a belt. This is commonly known as a Front End Accessory Drive (FEAD). This can be done with a V-Belt or Serpentine belt. This article will focus on the serpentine belt with an automatic belt tensioner.
Using a serpentine belt to drive your compressor has the benefits of lower maintenance, no slipping belts, no risk of over tensioning. A belt drive is torque dampening and a ‘slip point’ if something goes wrong with the auxiliary components. The downside is that the belt must be aligned more accurately than that of a V-Belt.
When designing your FEAD-driven compressor there are a number of points to consider:
Available power from the engine
We’ve talked about power requirements in a previous article. When designing your belt drive, the alternator, fan and water pump as well as any additional belt driven components, will need to be taken into consideration.
The speed of your drive engine as well as the ratio of your pulleys are an important consideration. Pulleys act as speed multipliers and the final ratio at your accessories needs to be understood. Running an engine at high rpm and using a high ratio belt drive system can have adverse effects on the bearings being driven.
Although a simple concept, the amount of space available for your compressor is another important factor. Some equipment may have exhaust components or fuel pumps in the way. For others, the existing sheet metal and packaging will have to be considered.
Adding a bracket to mount your accessory seems like an easy step. Finding the right mounting holes on an engine block or existing bracket is sometimes a challenge. Once you locate those holes and design your bracket, proper vibration and stress analysis will ensure you’ve got a solution to meet your needs.
Belt Load and type of belt
It is important that when selecting the type of belt for your application, you take into account the manufacturer’s specifications for each belt. Factors to look at are: the type of motor and its rated horsepower, the rpm range that the motor will be operating at, what accessories will be used and the associated loads, as well as the speed they will need to operate at and the environment in which the entire application will be running. The heat, oil and chemical resistance of the belt, should also be analysed for the particular environment.
To limit load pulsations, some FEAD designs use a flywheel to provide the offsetting flywheel effect. In some cases, the accessory you are driving will produce its own flywheel effect. The diameter of the pulley being used has a great effect on the flywheel effect. Weight, although to a lesser degree, is also a factor; work with the manufacturer of the accessory and engine to determine how much, or if you require flywheel effect for your particular application, or if your accessory is producing its own, this will need to be accounted for in your design.
Belt (pulley) wrap
Wrap is important for tension control on your belt drive. Using a wrap near 180 degrees is ideal. Using a smaller degree wrap has negative effects as when the tensioner arm moves, the wrap angle becomes even less.
Hub (bearing) load and overhang
Hub or bearing load is determined by how much tension is on the belt along the drive center line. Too much load or side loading can be detrimental to the longevity of the drive system. If the belt sheave is mounted beyond the bearing, your bearing load will increase depending on the distance the sheave is from the bearing. Having loads as close to centered over bearings as possible minimizes the overhang load. Hub load can change depending on the variable loading of the component the pulley is on, or any other load after the pulley on the belt line. For example, an air conditioning pump will add a load to the alternator if it is after that component in the belt line; this only happens once the air conditioning is turned on and the electric clutch is engaged.
Load capacity of bearings and speed ratings
Bearing selection for pulleys is critical for your FEAD design. The first pulley sees all the loads from the component after it, as well as the load induced by the component it is mounted on. The order of components is very important, as is considering the peak loads of each component. Axial and radial loads, static load carrying capacity, dynamic load carrying capacity, speed or rpm as well as calculating Mean Time Before Failure are all factors to consider during your design. More on bearing life expectancy can be read here.
Dynamic accessory torque and moment of inertia
Understanding the mass inertia of the various driven accessories is important. The effective inertia on the tight (drive) side is much lower than that of the slack side. Locating heavy loads on the tight side helps the motor handle those loads.
Automatic belt tensioners
Accessory Drive Tensioners provide constant tension to the belt to eliminate slip or noise. Belt vibrations are dampened and the tensioner automatically adjusts for belt stretch and wear, they also help maintain belt alignment. Automatic tensioners need to be the last component in the belt line so that they are on the “slack side” of the belt. The belt line should be designed to have the tensioner in the designed center point of the travel to allow for belt length variation due to loading and wear, while also giving enough travel to install the belt when new.
Belt length is an important factor in determining how much tension the system is operating at. Too short or too long and your tensioner may not be in the ideal position. A belt that’s too long may not have enough grip to drive all of your accessories and may slip causing premature belt failure. Another factor in determining belt length and FEAD design is what belts are available, a tweak to your design may be needed to source an existing belt.
Obviously belt alignment is important in your FEAD design. A misaligned belt can cause premature wear, failure or the belt coming off the pulley. Generally, a serpentine belt can have up to ½ degree of misalignment before you encounter issues. If your belt is more than half a rib off, it will track off the belt line. Once your components are mounted to the engine, a laser alignment tool can be used to ensure your pulleys are aligned. This video from Gates describes one such tool and its operation.
A properly designed FEAD can provide you with years of worry-free service. Taking into account the points listed above will help you with your design process.
If you have any questions about this article or anything mobile compressor related, please contact us.