Internal Combustion Horsepower Ratings

When designing a belt drive with an internal combustion (I.C.) engine it’s important to understand that an IC engine is not like an electric motor. The horsepower rating for a NEMA motor is not the same as a horsepower rating for an IC engine. There is no conversion factor. HP is a HP is a HP. Engines just have different characteristics. Electric motors may have a hard start whereas IC engine may not. IC engines can’t run at their peak torque for a long time. An IC Engine with a HP rating higher than the power needed should be selected. Altitude and other factors can affect an IC engine’s performance.

Internal combustion engines are typically rated based on brake horsepower (BHP), or maximum BHP.

This BHP rating of an engine usually means the horsepower produced by a test engine in a laboratory.  During the test an engine is ran without a fan, generator and other accessories.  The ambient temperature is corrected to some standard, such as 60°F., and the atmospheric pressure is corrected to some given altitude, such as sea level.  The BHP rating should not be used for design, since the standard production engine, with accessories, cannot reach this output in actual usage.  Gross BHP is the term used for test data without any accessories and net BHP is with all of the standard accessories.  This is the horsepower measured at the crankshaft flywheel.  An I.C. engine spec will not contain any other mention of horsepower.

Several decades ago it was more common to refer to a maximum intermittent HP.  For short durations this was generally 85% of BHP.  Continuous duty or rated BHP was 75%-80% of maximum BHP for long duration service.  These terms are not referred to today in engine specifications.

It is still important to verify the horsepower versus engine rpm curves for those applications where the engine and drive are not intended to run at one speed continuously.  A percent time duty cycle is also helpful in selecting a belt drive.

Actual HP

Do you know that people are constantly using bigger motors than necessary? It can be a big problem, and doing so can lead to using bigger belt drives than necessary too. There is a common belief that bigger is always better. This is not always the case when it comes to power transmission components. Designing around actual loads not only saves on initial purchase price, but can also save money on replacement parts caused by excessive tensions, such as those placed on bearings when too big of a drive is used.

If you think your motor is too big for the load you are using, here is an easy way to calculate actual HP draw.

Actual HP = (Nameplate HP)x(Measured Amps) / (Nameplate Amps)

This means that if you measure the amperage draw of your motor, you can use the nameplate to find out what your actual HP draw is. Now sometimes it’s worth it to design around the rated load instead of the actual load. In situations such as hard starts, or unknown shock loads, having additional service factor is good, but when max loads are known, or size/cost is a priority, we can use the above info to get just the right size drive.

Tension Gauges: Pencil vs. Krikit

We all know that tension is important, and measuring tension with a gauge is a great way to prevent belt drive problems. However, there are a few options out there, and sometimes people are confused as to what they need.

Two of the lower cost options that Gates offers for tension testers are the Pencil Type Gauge, and the Krikit Gauge. Both of these are easy to use tools that allow the user to measure tension in the belt, and compare it to recommended tensions, but they function a little differently.

The Krikit gauge is generally seen as an automotive gauge used on front end accessory drives for cars and trucks. The way this gauge works is by depressing the finger pad on the gauge with the bottom of the gauge against the belt. The belt will deflect downward and push the arm of the Krikit up across a scale on the top of the gauge. At a certain amount of force applied to the finger pad, the Krikit will ‘click’. When the user hears the click, pressure should be released, and you can read the amount of tension in the belt by looking at where the front of the arm crosses the scale.

The Pencil gauge is generally seen as an industrial gauge, and uses two o-rings and a spring. You place the big end of the gauge on the belt, and set the bottom o-ring to the recommended deflection distance. You will need a straight edge, piece of string, or a mark on the wall next to the belt to determine starting height of the back of the belt. Set the plunger o-ring to zero, and push down on the plunger until the bottom o-ring meets the reference point you set for the starting height of the back of the belt. At this point, release the pressure on the plunger, and read the force recorded by the movement of the plunger o-ring. This is the value that you will want to compare to the recommended tension values for your drive.

As described above, there are obvious differences in how the gauges function, but the one thing that may not be obvious is the tensions that they are reading. The Krikit measures tension ‘in’ the belt, while the pencil gauge measures deflection force at a certain distance. This is important to note because different sources will recommend tension differently, either direct tension that the belt is seeing (the tension in the belt), or as a matter of deflection force and deflection distance.

Both of these tools are offered in several capacities for measuring tension, and both work very well, but they work in two different ways. It’s important to know which way your tension measurement is being given so that you can select the proper tool.

V80 and Belt Matching

A lot of people call us asking about a matched set of V-belts. This is an important design aspect, as drives that use multiple V-belts have to have belt lengths that are close to work properly. Back in 1980 Gates made a change to their manufacturing process that allowed us to meet RMA V-belt matching standards with our standard line product in the Super HC, Hi-Power II, and Tri-Power belt lines. Using any of these belts made to our V80 standard means that you can use off the shelf belts of the same size and not worry about matching them. This can save considerable time trying to find a set of belts.

We do have product lines that are not V80 approved, and do require belts to be matched. Our Predator line of V-belts are a good example. Because of the Kevlar tensile cords used in Predator, matching the belts to the same punch number is required.

Benefits of the Gates EZ Align Precision Laser Tool

Accurate pulley alignment is very important in maximizing the performance and longevity of belt drive systems. Measuring and correcting misalignment is not always easy, though, so laser type tools can be very helpful. While a number of different types can be found, the Gates EZ Align Precision Laser Tool is definitely a best in class device and should be seriously considered.

EZ Align cases are made of tough machined aluminum with a durable finish allowing survival in industrial environments and in tool bags. Strong rare earth magnets mount emitter and target units securely. The lasers are bright with high quality optics, and units can be calibrated for accuracy or repaired as needed. The new EZ Align Green model uses an even brighter green laser for outdoor use in bright sunlight. The EZ Align Green laser is 10 times brighter than the standard EZ Align laser and can reach up to 15 feet.



The EZ Align Tool is capable of indicating misalignment in three different planes.  The reflective method used in the measuring process is what really sets the EZ Align apart from other laser alignment tools.  Reflecting the beam from the target unit back to the emitter unit multiplies angular misalignment making it highly visible for accurate correction.  Angular misalignment is multiplied 20 times greater than other non-reflective laser tools.  Types of misalignment indicated by the EZ Align Tool are illustrated below:

Laser alignment tools are invaluable in accurately indicating pulley alignment and should be a part of every preventative maintenance program.   Gates EZ Align and EZ Align Green tools are the best available and are well worth their cost.

Self-Generated Tension

All synchronous belt drives exhibit a self-generating or self-tightening characteristic when transmitting a load. Laboratory testing has shown this characteristic to be similar with all tooth profiles. The designer/user should be aware that self-tensioning can result in increased bearing and shaft loads and reduced drive performance; i.e., short belt life. This can be avoided by following proper tensioning procedures.

While belt overtensioning can impose higher bearing and shaft loads and lead to reduced belt life, undertensioning can result in self-tensioning. Properly designed and tensioned drives will not be significantly affected by self-generated tension.

When a belt is too loose for the design load, the self-tensioning characteristic results in the belt teeth climbing out of the sprocket grooves, leading to increased stresses on the belt teeth, accelerated tooth wear and reduced belt life. When a belt is severely undertensioned, this self-tensioning characteristic can result in the belt ratcheting (jumping teeth). When this occurs, significant shaft separation forces are instantaneously developed in the drive, resulting in damage to bearings, shafts, and other drive components including the belt.

NOTE: This is true for all synchronous belts.

Maximum drive performance and belt life are achieved when the belt is properly tensioned for the design load and maintained.

Belt Dressing

Gates does not recommend applying dressing to belt drives. While belt dressing may temporarily quiet a slipping V-belt drive, it only masks the real problem (i.e. low belt tension). If a belt is slipping, it should be re-tensioned.

Additional information on drive inspection and troubleshooting is available in the Belt Drive Preventive Maintenance & Safety Manual at http://www.gates.com/brochure.cfm?brochure=1224&location_id=3288/.