A Guide To Calculating Disc Spring Fatigue

You should always think about the wear and tear of your manufacturing components in order to ensure continuous progress. Here’s what you need to know about calculating disc spring fatigue.

A Guide To Calculating Disc Spring Fatigue - KB Delta

Disc springs are quite unique since the deflection for a given load and the minimum fatigue life can be predicted using standard calculations outlined in DIN EN 16984. As such, engineers can calculate the minimum fatigue life for disc springs for a given load.

On the other hand, disc springs possess a high capacity to store potential mechanical energy, consistently. They are also able to provide more force in less space when compared with wave spring or compression spring.

Here’s a guide to calculating disc spring fatigue.

What are Disc Springs?

Disc springs are washer-like components that are conically shaped. Their conical design aids in an easier prediction of their spring characteristics and performance, in comparison with traditional compression springs.

On the other hand, disc springs are meant to be axially loaded. They can be loaded dynamically, or they can be statically loaded continuously or intermittently. These components can either be used in singles or in multiples in a bid to achieve application-specific spring rates.

What’s more, disc springs can be stacked in series, parallel, or a combination of both. Each spring’s precise tolerance provides predictable performance when they are stacked.

When stacked in series, less force is provided over more travel, while a stack in parallel gives more force over less travel.

Application of Disc Springs

There are various areas disc springs are applied and some of these are:

  • Brake systems
  • Aircraft landing gear
  • Clutch assemblies on power tools
  • Industrial piping support systems
  • Pick-off spindles in CNC screw machines

In the case of brake systems, they are hydraulically actuated or caused to operate. Here, pressurized fluid leads to the compression of stationary friction discs against plates that rotate in line with the driveshaft.

Each set of plates has an amount of friction, which helps to control deceleration.

Though, the brakes may fail if pressure is lost from the hydraulic cylinder or the hydraulic seal is compromised.

However, it is worth noting that disc springs provide a mechanical back-up system. For disc springs stacked in series, the hydraulic system holds a constant pressure. A failure in maintaining this pressure causes the stack of springs to decompress and, therefore, actuate the braking mechanism.

Advantages of Disc Springs

There are several advantages of disc springs over other types of springs and some of these are:

  • It offers longer fatigue life
  • Parallel stacking offers inherent damping
  • It has consistent performance under design loads
  • It provides high load capacity with small deflection
  • Offers several load and deflection characteristics

How to Calculate Disc Spring Fatigue

The ability to predict the deflection of a spring at a given load makes it easy to calculate the force and stress levels on the disc. The more the disc spring flexes, the higher a change in its stress level which could cause a faster disc spring fatigue.

There are, however, tensile stress points that can be used in determining the fatigue life of disc springs.

Accordingly, the fatigue life can be estimated by evaluating the maximum stress difference between the preload and final load at two locations. Locations that have the highest stress differential are then used to predict the disc spring’s fatigue life.

Likewise, fatigue life charts for springs in DIN EN 16983 are also useful in estimating the fatigue life of the disc spring.

The Fatigue life charts serve springs whose thickness is in the following range, not above 1.25 millimeters, around 1.25 and 6 millimeters, and between 6 and 14 millimeters.

Calculating Fatigue Life of a Disc Spring

The outlined above can be used to estimate the fatigue life of a disc spring.

For instance, if such a spring has an inside diameter, outside diameter, and thickness of 25.4 millimeters, 50 millimeters, and 2 millimeters respectively. And the preload is 15% of the initial height of the spring, while the final position is 75% of its initial height. The stress at one location such as location II (σII) can be around is 128 newtons per square millimeter (N/mm2) at 15 percent of the spring’s initial height.

Also, at 15 percent of the spring’s initial height, the stress at another location such as location III (σIII) can be 264 N/mm2.

The stress at σII is 923 N/mm2 and the stress at σIII is 1,140 N/mm2, at 75 percent of the spring’s initial height.

This data can then be used to find the stress at each location. The calculation would be as follows:

  1. Location II: 923 – 128 = 795 N/mm2.
  2. Location III: 1,140 – 264 = 876 N/mm2.

Therefore, the maximum differential in stress is at Location III.

Accordingly, the fatigue life of the disc spring can be estimated using stress values from location III and the fatigue life charts. In order to use the chart, a vertical line representing the minimum stress at location III can be drawn on the X-axis.

In addition, a horizontal line representing the maximum stress at location III can be drawn on the Y-axis. The estimated fatigue life, therefore, becomes the point where these two lines intersect.

Meaning of Results

Using the data above, the intersection is expected to be above the 100,000 cycle line if the line on the X-axis is at 264 N/mm2 and the line of the Y-axis is at 1,140 N/mm. To that effect, the intersection shows that the estimated fatigue life of the spring would be a bit less than 100,000 cycles.

The result shows that a reduction in deflection causes an increase in fatigue life.

Much more, the deflection range of a disc spring determines its fatigue life and an increase in the final load increases the stress in the disc spring which brings about a lower fatigue life.

It is worth pointing out that fatigue life charts are linked to laboratory tests on single discs at room temperature.

Also, the test is done at a frequency to prevent heat build-up, and the test discs are lubricated before being tested on polished anvils. But, the actual fatigue life may differ from the values estimated in the fatigue life charts.

A Guide To Calculating Disc Spring Fatigue - KB Delta

Conclusion

A guide to calculating disc spring fatigue ensures that you can properly estimate the fatigue life of your spring.

As such, you can ascertain how long these components will last and serve well in your application.

The process involved in determining the disc spring fatigue is quite easy; hence, whether you are a spring manufacturer, or a company looking for the best springs, these may prove useful.

 


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