Timing Belt Tension Guide

Improperly tensioning a timing belt at installation is one of the most common causes of timing belt problems, leading to issues ranging from tooth skipping to tensile fatigue, and in some cases complete belt failure.  

To maximize a drive’s performance and lifespan, proper tension is essential. In this guide, we break down the importance of timing belt tension, three ways to measure and set pretension, and adjustments to make to get the tension right.  

Why timing belt tension matters

Timing belt tension is critical to achieving optimal performance and getting the most out of a drive. An improperly tensioned timing belt often results in increased noise, precision loss, accelerated wear, and can cause timing belts to break. In short, if a timing belt isn’t tensioned correctly, it likely won’t work the way it needs to.  

When people discuss timing belt tension during installation, maintenance, or troubleshooting, they are usually referring to the initial pretension, which is the tension applied when a belt is installed, or static tension, which is the tension when the belt is not moving.  

Pretension significantly influences how a belt behaves under load, so issues with the operating tension can sometimes be addressed by adjusting static tension.

3 ways to measure timing belt tension

Measuring timing belt tension is a critical step in installing a timing belt. There are three main ways to measure static tension of a timing belt:

• Using a sonic tension meter
• Force deflection test
• Elongation test  

Tension meter

The SM5 Tension Meter is the easiest and most accurate way to measure and set belt tension. The sonic belt tension meter measures the frequency of belt vibration, which can be used to calculate the current static tension using the formula below:

If you already know the pretension value you need to achieve, you can use that to calculate a target belt frequency measurement and compare that to the reading on the meter using the following formula:

When you use the SM5 Tension Meter, you can pair it with our free SM5 Pretension Calculator App that performs calculations instantly, eliminating the need to manually complete complicated formulas or reference external charts.

Force deflection test

Tension can also be measured by performing a force deflection test, in which force is applied to the center of the belt span, and the deflection distance is measured.

The force and deflection measurements can then be used in a series of calculations or compared to specifications provided by the belt manufacturer to determine if the belt is over or under tensioned. Our Applications Engineering team can provide details and formulas when required to assist with force deflection measurements.

Force deflection tests are typically done using a pencil force gauge to get an understanding of how much force is applied to the belt. However, even with a force gauge, this method is not as accurate as using a sonic belt tension meter.

Elongation test

Elongation tests can also be used to determine belt tension, which is typically used for longer belt spans or linear drives. When a belt is tensioned, it stretches slightly. By identifying how much a belt should elongate with proper tension, you can get a general idea of how to install the belt.

An elongation test for belt tension starts by measuring the belt span and calculating the estimated elongation under proper tensioning, which varies by tension member material.  

• Steel cords elongate by an estimated 0.4%
• Kevlar reinforced belts elongate by an estimated 0.66%

Once you have the elongation distance calculated, you can install the belt by marking the tensioner and a fixed point with the appropriate distance and moving the tensioner by that distance.  

Like force deflection tests, elongation tests are not the most accurate method for measuring belt tension, especially when compared to using a tool like the SM5 Tension Meter.

How to adjust timing belt pretension

There are a few different ways to adjust pretension when a timing belt needs more tension:

• Moving the pulleys: In systems where pulleys can be adjusted, moving them farther apart to increase center distance can add more tension to the belt. This method is most popular for conveyor applications.  ‍
• Adding a tensioner: When pulleys cannot be adjusted, a tensioner idler pulley can be added either inside or outside the belt to apply additional force by altering the belt path. ‍
• Use a tensioning clamp: Adding a tensioning clamp in a linear drive system provides more control over belt pretension.

If a timing belt has too much tension, try adjusting clamps and the position of any tensioners or decreasing the pulley center distance, if possible. In some cases, an over-tensioned belt could indicate that the belt is too small for the application, and a new belt is required.

What happens when timing belt tension is too tight?

An over-tensioned timing belt can lead to wear, noise, and increase the risk of failure. Here are a few common timing belt symptoms that may indicate that tension is too high:

• Tensile failure or fatigue: Too much tension can put excess stress on tension members within the belt causing them to snap or break.  ‍
• Accelerated tooth wear: When a belt is wrapped too tightly around a pulley, it can impact how the teeth mesh with the pulley grooves and increase wear.  ‍
• Overheating: Friction can increase and generate excess heat when a timing belt is over-tensioned, which can weaken the belt and accelerate wear. ‍
• Whining noise: Unusual whining noises can occur when belt tension is high, though it’s not the only cause.  ‍
• Bearing and shaft damage: Excessive tension can put a lot of stress on the surrounding components as well as the belt itself, causing premature wear, uneven loading, and misalignment.  

What happens when timing belt tension is too loose?

When timing belt tension is too low, it can cause damage to drive components and impact overall performance.  

• Reduced precision and accuracy: Under tensioned belts deflect more underload and have delayed motion response, which directly impact precision and positioning, and can end up impacting product quality.  ‍
• Slapping noises: Timing belts making slapping noises often indicates that there is not enough tension in a drive.  ‍
• Tooth skipping: If a timing belt isn’t tight enough, it won’t engage properly with the pulley grooves, potentially accelerating tooth wear, decreasing precision, or causing sudden shock loading that could break the belt.

Don’t slack when it comes to timing belt tension

Many timing belt issues are the result of improper installation, not the belt itself, which is why it’s so important to get the pretension right from the get-go. Otherwise, you run the risk of premature wear, diminished performance, and shortened lifespan, all of which will cost you time and money in the long run.

The easiest and most accurate way to measure static belt tension is with the SM5 Tension Meter, which can be used across belt types, regardless of tension member material. Other tests, like the elongation or force deflection methods, can also be used to measure timing belt tension but are far less precise than using a sonic belt tension meter.  

The correct pretension for a timing belt depends on the belt and drive design. If you’re unsure of the correct tension and frequency values for your system, you can contact our Application Engineers for free tensioning support.

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