Know When, Not If, You Need To Replace Your Linear Screw Actuator

Linear screw actuators deploy mechanical components including but not limited to: ball bearings, ball screws and roller screws. These components experience wear and tear through their usage. It is only a matter of time before linear screw actuators reach the end of their integrity.

Most actuator manufacturers should be able to provide the DLR (dynamic load rating). The DLR is a value measuring the constant load that allows a ball-bearing device to reach 1,000,000 actuations. It is measured by factors such as screw diameter, lead, and the number of movable parts (balls or rollers) that also carry load.

Without this quantity, the life of your device cannot be properly measured – and you don’t want to be taken by surprise by a faulty actuator in the middle of a job.

The estimation is a fairly simple task, if you use the L10 life formula for ball bearings. This calculation hones in on the rolling elements as its the most critical moving component. To be able to make such calculations, however, there are some terms and definitions you should be made aware of:

Terms You Need To Know:

You already know what DLR is – a number that gives you an estimate as to how long a ball bearing device will last under constant load, while achieving 1,000,000 rotations. Typically, it’s provided by your actuator manufacturer.

Constant load is the load that does not change during the working cycle. Varying load changes throughout the working cycle.

When an application has a load that varies throughout the working cycle, you have to calculate the overall total load acting on the screw. You are measuring the equivalent dynamic load, which is the measurement of what the approximate wear and tear would be if the load had been applied constantly.

The time you can expect 10% of the bearings in the application to fail due to normal wear and tear is called L10 or B10 Life.

You can use these definitions to help calculate the estimated life of your actuator. The calculation you make, while not a guarantee, will tell you how long you should be able to expect the application to perform without replacement. Eventually, normal wear and tear will require replacement – but if you make an accurate calculation, you can properly estimate when your linear screw actuator will need replacement.

Calculate L10 Life with a Constant Load

The formula for estimating life with a constant load is as follows:

L10 = (C/Pe)3 x l

C = Dynamic load rating (lbf) or (N)

Pe = Equivalent load (lbf) or (N

l = Screw lead (in/rev) or (mm/rev)

For example, in a case where:

C = 10,000 lbf

Pe = 5,000 lbf

l = 5 mm

L10 = (10,000/5,000)3 x 5 =

(2)3 x 5 =

8 x 5=

40 =

40 million mm

Life With a Varying Load 

If during your working cycle the load varies, you will need to calculate the equivalent dynamic load using the following formula:

Pe = 3 √{ [L1(P1)3+L2(P2 )3+L3(P3 )3+Ln(Pn )3]/L}

Pe = Equivalent load (lbs) or (N)

Pn = Each increment at different load (lbs) or (N)

L = Total distance traveled per cycle (extend + retract stroke) (L = L1 + L2 + L3 + Ln)

Ln = Each increment of stroke (in) or (mm) at different load

Then you can use the calculated Pe in the life calculation formula:

L10 = (C/Pe)3 x l

Estimating Actuator Life

Here are the variables:

L10 = Life calculation in distance

L = Total distance traveled per cycle (extend + retract stroke)

CpM = Number of cycles per minute

HpD = Number of hours operated per day

DpY = Number of days of operation per year

Put that information into one of these formulas:

Life Estimate in Years =

(L10 / L) / [(CpM) X 60 min/hr X (HpD) X (DpY)]

Life Estimate in Days =

(L10 / L) / [(CpM) X 60 min/hr X (HpD)]

 

Still have questions about your actuator? Contact GCC to learn more!

 

The above information was provided by our world class supplier Tolomatic.