How to measure the force exerted by a spring?

In fact, all springs are designed to generate a specific force within a given housing; it is therefore essential to verify the calculations—and, above all, the manufacturing parameters—by measuring the forces.

In most cases, the spring is placed in a mechanism that has a rest position and an operating position. A spring is therefore defined by specifying two operating heights, H1 and H2, and measuring the expected loads when the spring is compressed to H1 and H2. The required test thus consists of measuring the forces exerted by the spring at H1 and H2.

For a standard spring, the tolerance range is 10%; for a so-called precision spring, it is reduced to 5%.

Several factors will influence the quality of the measurement:

• The geometry of the spring:
  
  

Coil concentricity: The spring’s axis must be centered.

Perpendicularity of the bearing surfaces. In particular, better repeatability is achieved with a spring whose surfaces are ground and whose last coil is flush.

Height-to-diameter ratio: A “slender” spring will tend to bend during compression (buckling). In this case, it must be guided to ensure it remains vertical. Internal guide pins are generally used.  These pins must be carefully designed so that friction between the spring and the pin at the moment of buckling is minimized and, above all, remains constant.

Internal stress level: Some springs are designed at the limit of the allowable internal stress levels. In this case, during the first compression tests, the metal enters the plastic deformation range and the spring “stretches” slightly. The values obtained during the first test will be higher than those from the third or fourth test. It is generally recommended to perform a few tests with the coils in close contact (often 5) before taking measurements to avoid these variations.

• Manufacturing parameters :

The consistency of the wire's characteristics, which can vary within a single coil but especially from one coil to another.

The consistency of the spring-coiling machine settings.

The stresses on the wire during spring forming, which sometimes make it necessary to “relax” the spring before measurement.

• Characteristics of measuring instruments:

Plate geometry: The plates must have a perfectly flat surface, and both plates must be perfectly parallel.

Sensor resolution: The specifications of the force and displacement sensors (resolution, accuracy) are, of course, critical factors.

For the force sensor, the accuracy typically required is 0.1%, and the resolution must be finer than 1/1000.

For the displacement sensor, a resolution of 0.01 mm is sufficient, except in the case of very small springs or springs with low loads. Particular attention must be paid to the accuracy of the displacement sensor when dealing with high-stiffness springs. In fact, in this case, a small error in the displacement measurement will result in significant variation in the force measurement.

The overall quality of the sensor mounting frame—and especially its rigidity.

Frequently Asked Questions About Spring Measurement:

• What is cell compensation?

This term refers to a process that corrects errors in displacement measurements caused by deformation of the frame on the one hand and deformation of the sensor on the other.

Using very rigid frames and high-quality sensors significantly reduces this error. A well-designed system will have an error of less than 0.15 mm. However, in the case of very precise measurements, it may be necessary to perform this operation.

Electronic method: Some electronic systems incorporate correction using a coefficient calculated by the manufacturer. This method does not allow the coefficient to be adjusted to account for wear and aging of the frame and the sensor.

Mechanical method: This method involves placing the plates in contact, applying a force equivalent to that of the springs being measured, and zeroing the displacement sensor.

Should positioning stops be used to determine heights H1 and H2?

In addition to the difficulty of adjusting the stop positions, it is important to keep in mind that mechanical stops are subject to variation depending on the approach speed—or even on changes to the adjustment itself. This method is reserved for situations involving the measurement of very large batches by entry-level operators.

An experienced operator will obtain more consistent and faster results by using displacement sensors to position the probe at H1 and H2.

Some software programs capture the force on the fly as the values transition to H1 and H2, without the need to stop. This is a very effective method, provided that the associated electronics have a high sampling rate so that the force can be “captured” as close as possible to the desired height.

What is a “spike” measurement? 

Sometimes we want to know the value of the spring force for a given displacement rather than for a given spring height; this is called the “deflection” measurement.

This measurement is similar to the measurements at H1 and H2, but in this case, the height H1 is the free height of the spring. Generally, zero displacement is defined as the point at which the upper plate comes into contact with the spring.

This method is quite commonly used for torsion springs.

How to measure the ground clearance?

To measure the spring’s free length reliably and reproducibly, a minimum force to be reached is determined, and the displacement value at that point is recorded.

To avoid any issues related to measurement noise, it is recommended to set this force to three times the sensor’s resolution.

How to Measure the Block Height?

The term “fully compressed height” refers to the height of the spring when it is compressed until all its coils are touching. In this configuration, the spring behaves like a solid block of metal. It is dangerous to fully compress a spring using a spring testing machine because the machine’s capacity limit is reached very quickly; the force increases exponentially as soon as the spring is “fully compressed.”