Understanding the use of a tensiometer on a sawblade

Introduction

In order to perform well, the blade of a bandsaw must be properly tensioned. If it is too loose, the cut will not be straight, and the blade may come off its wheels due to the force applied to advance through the wood. If it is too tight, the lifespan of the blade and other mechanical components of the sawmill will be shortened. Additionally, excessive tension in the blade can cause it to break during use.

Typically, the blade is mounted on two wheels. The leading wheel is driven by the motor. The other is called the driven wheel. The tensioner applies a force to the axis of the driven wheel. Thus, the force applied by the tensioner is double the tensile force in the blade.

To properly use a tensiometer, it is important to understand the following principles:

1- Force F: Force is expressed in pounds, kilograms, or Newtons.

2- Stress σ: Stress is the force per unit area, expressed in pounds per square inch (PSI), kilograms per square meter, or Pascals.

3- Strain L: Strain is the elongation of the blade, expressed in inches, centimeters, or thousandths of an inch.

4- Section A: The section is the perpendicular area obtained by cutting the blade at a right angle. The section is expressed in square inches or square meters.

According to these definitions, if a tensile force of 1000 pounds is applied to a 1-inch by 1-inch square bar, an internal stress of 1000 PSI will be produced in the bar. The same force applied to a ½ inch by ½ inch bar will produce a stress of 4000 PSI.

Therefore, stress σ is equal to the force divided by the section = F/A.

Tensiometer

A tensiometer is a device that allows the evaluation of the force or stress present in a material. A tensiometer that measures force is based on the angular deformation of the blade. A tensiometer that measures stress is based on the longitudinal deformation (elongation) of the blade.

The elongation tensiometer measures deformation (stretching) over a certain reference length. It thus provides a stress reading. It is important to note that the tensiometer does not measure force (i.e., pounds) but rather pressure in PSI (internal stress). Secondly, this pressure obtained on the tensiometer is unrelated to the pressure read on the pressure gauge (also in PSI) found on the blade tensioner of certain sawmill models.

Elongation tensiometer

Useful Theory for an Elongation Tensiometer

In the elastic deformation zone of steel, a constant allows the relationship between the internal stress in the blade and the tensile force according to the measured deformation. This constant is called Young's modulus. Young’s modulus of steel is a constant equal to 29 million PSI.

Young’s modulus "E" is defined as follows:

E = F / A * Li / (L - Li)

Where:

E = Young's Modulus = 29,000,000 PSI

F = Applied tensile force in pounds

A = Cross-sectional area of the blade in square inches

L = Length of the blade when tensile force is applied

Li = Length of the blade before applying tensile force. For an elongation tensiometer, Li is a constant defined during the design of the device. It is defined here as 5 inches due to lack of exact value.

Blade manufacturers provide the nominal stress to be respected when using the blades. They obviously leave a margin to avoid reaching the elastic limit of the saw blade. This is the maximum stress (not force) that a material can endure without permanently and irreversibly deforming. For a sawmill blade, this value should obviously not be reached, or else the blade will stretch and must be discarded.

The typical nominal stress for ordinary steel used in sawmill blades ranges from 15,000 PSI to 20,000 PSI, while it reaches 25,000 to 34,000 PSI for high-quality steels (bi-metal, spring steel, or carbide).

These limit values constitute the maximum F/A ratio (also in PSI) that the blade can withstand.

Since high-quality blades are more expensive, they are rarer. In the absence of information about the steel grade of a blade brought by a customer, it is wiser to perform calculations using the average between 15,000 and 20,000 PSI, which is 17,500 PSI.

With this, we finally have everything needed to determine the tensile force in a blade by measuring its elongation.

And the Elongation Tensiometer in all this?

In the following equation, the instrument allows the measurement of L. The value of Li (i.e., the manufacturer considered it in the design of the instrument) is known, which is the distance between the two anchor points. The dial integrates Young's modulus so that a pressure value can be read directly:

F/A = (L - Li) / Li * 29,000,000

And, by calculation alone, the force can be obtained:

F = (L - Li) / Li 29,000,000 A

Examples:

Example 1:

The elongation tensiometer gives a value of 17,500 PSI on a blade 1.25 inches wide (1 inch at the narrowest part) and 0.042 inches thick. What is the tensile force applied to the blade?

F/A = 17,500, so F = 17,500 * A

A = 1 * 0.042 = 0.042 square inches

Therefore, F = 17,500 Lbs/in² * 0.042 in² = 736 lbs

Example 2:

The elongation tensiometer gives the same value of 17,500 PSI on a blade 1.5 inches wide (1.25 inches at the narrowest part) and 0.045 inches thick. What is the tensile force applied to the blade?

F/A = 17,500, so F = 17,500 * A

A = 1.25 * 0.045 = 0.05625 square inches

Therefore, F = 17,500 Lbs/in² * 0.05625 in² = 985 lbs

The reader will understand that a larger blade undergoes a greater tensile force if it is stretched by the same amount. These two examples clearly show that the same value read on the elongation tensiometer gives two different forces for blades with different dimensions. This confirms that the pressure read on the pressure gauge of the tensioner cannot be directly correlated with the value read on the elongation tensiometer without considering the blade dimensions.

Considerations for Sawmill Construction

There are two things to consider here:

1- The rigidity of the structure supporting the blade wheels.

2- The design of the blade tensioner.

Sawmill Rigidity:

As demonstrated earlier, if a larger blade is installed on a sawmill and the stress in the blade is measured with an elongation tensiometer, the frame supporting the blade wheels can break because the tensiometer will give the same value, but the tensile force is significantly larger.

Likewise, the shafts of the wheels will be much more stressed. This can lead to faster fatigue failures of the shafts.

On the other hand, if the same force is always applied to the sawmill tensioner with a smaller blade or a blade that has been sharpened multiple times, the blade may approach its elastic limit and prematurely break. Internal forces that occur when the teeth contact the wood are disregarded here. Additionally, the tension in the blade is not uniform. The part of the blade going from the tree log to the leading wheel experiences additional force coming from the engine.

Therefore, it is crucial to understand the use of the tensiometer to avoid undesirable situations.

Sawmill's Tensioner Design

There are various tensioner designs. Some are screw-based, others use levers, and others are hydraulic. However, as we saw earlier, none of them can determine the absolute stress in the blade without using a tensiometer. At best, these designs allow the replication of the tensile force on the wheel axis.

In hydraulics, the force of a push cylinder depends on its internal surface area and the oil pressure applied. For instance, a 2-inch internal diameter cylinder with a 1000 PSI hydraulic pressure will push 3,141 pounds. The same 1000 PSI applied to a 1.5-inch internal diameter cylinder will push 1,767 pounds.

Thus, disregarding other mechanisms that may amplify the force of the tensioner’s cylinder, it is clear that the design of the tensioner is directly related to the force actually applied to the axis of the blade’s driven wheel. It is also important to remember that the force applied by the tensioner is double the tension in the blade.

In other words, 1000 PSI obtained on the tensioner of a given sawmill does not necessarily tension the blade with the same force as the same 1000 PSI on a different sawmill. Therefore, it cannot be relied upon to properly tension the sawmill blade.

Interpretation of the Tables Provided with the Elongation Tensiometer Document

The following table shows the elastic limit values (i.e., F/A) for different steel grades. These values are necessary to determine F/A and calculate force.

Additionally, the following table correlates the rigidity of the sawmill with the blade installed on it. As shown earlier, the larger the blade, the lower the value indicated by the elongation tensiometer should be to avoid damaging the sawmill. We also see that the manufacturer sets a limit to prevent damage to the sawmill.

Allowed maximum stress for different sawmills

Conclusion

1- Without the appropriate knowledge and calculations, using the elongation tensiometer does not yield the correct blade tension values.

2- The nominal stress of the steel in question must not be exceeded to prevent premature cracking in the blade core.

3- The rigidity of the sawmill imposes an additional limit on the tension of the blade.

4- The pressure indicator of the blade tensioner does not indicate the blade tension but allows for reproducing a previously measured tension with a tensiometer.

Discussion:

The length of the blade is irrelevant in using a tensiometer.

The geometry of the blade (presence of teeth) influences its linear deformation. The position of the elongation tensiometer on the blade is therefore important. The more teeth between the two anchor points of the elongation tensiometer, the more the tension in the blade will be misinterpreted for a given reading.

Effect of Elongation Tensiometer Position on the Blade

The literature provided with the elongation tensiometer is insufficient and does not allow the average person to obtain the blade tension.

Some sawyers report that when the belts on the wheels are worn, the frequency of blade cracking increases. While this is an established fact, there is no theoretical explanation for it except that the rubber belt changes (i.e., dampens) the resonance frequency and impact level in the blade. Vibration fatigue (and thus cracking) might be reduced, but this theory remains to be proven.

Other types of tensiometers exist. Those used to measure tension in boat mast guys, for example, are based on the angular deformation of the guy rather than its linear deformation. This approach significantly reduces the complexity of interpreting the value as it only involves a geometric force ratio.

The sawmill manufacturer must provide the limit of tension that the saw’s structure can support. In other words, just because a larger blade is installed does not mean it can be tensioned more.

After each sharpening, your sharpener should record the following information on the blade:

• The residual cross-sectional area of the blade core after sharpening in square inches.

• The tension force the blade can withstand to respect the nominal stress of 17,500 PSI.

• The deflection value if a TensHOPs tensiometer is used.

TensHOPs Tensiometer by 1200W Inc.

Angular Tensiometer

The TensHOPs tensiometer uses the principle of boat guy tensiometers. Through angular changes, the tensile force in the blade reduces the deflection created by springs. Measuring this deformation is directly related to the tension in the blade.

Each TensHOPs is factory calibrated for accuracy.

The tensiometer comes with the following table that directly links the deflection measurement to the blade tension:

Advantages of the TensHOPs Tensiometer over an Elongation Tensiometer

• No need for complex calculations in the field to properly tension the blade.

• Up to a certain point, deflection is independent of the blade size. A sawyer mixing blade sizes will consistently obtain the correct blade tensions without paying close attention.

• If the engine power and pulley dimensions are provided, these can be integrated into the table to yield a better tension value, thus increasing blade lifespan. For instance, a 6-inch diameter pulley mounted on an 18 hp motor driving a 20-inch diameter wheel requires a 59-pound reduction in blade tension. This means that the more powerful the engine, the lower the blade tension needs to be.

• Even without its digital indicator, the TensHOPs can provide a good estimate of the blade tension. Thus used, it can serve well to replicate the tension after changing the blade. It should be remembered that the elongation tensioner cannot do this.

In addition to its greater ease of determining the tensile force in the blade, the TensHOPs is also significantly less expensive. It costs a third of the price of an elongation tensiometer.

Its measurement is not affected by the position of the tensiometer on the blade.

[1] It is important to note that the blade section varies between the two anchor points of the tensiometer, so we assume that Wood-Mizer has accounted for this in their measurement scale. The blade weld should also not be between the two anchor points of the tensiometer.

P.S.: There are two other texts dealing with different aspects of the topic in the BLOG section of the website www.1200W.CA.