Estimating how long a piece of equipment will last in an application is critical information in the specification process. This step-by-step guide examines how to estimate the life of electric actuators that use either ball or roller screws in applications with constant or varying (changing) loads. In addition, it explains how to calculate life in units of hours, days, or years, and how to compare the expected life of two different actuators.

Load is just one factor that affects an actuator’s service life. Factors that shorten an actuator’s life or lead to premature failure include incorrect mounting/alignment, lubrication starvation, high temperatures, and other environmental extremes. Consider each of these factors to ensure a long service life and maintenance-free operation for your application.

**Dynamic load rating** **(DLR) **is a constant load (in direction and magnitude) which lets a ball-bearing device reach 1,000,000 revolutions of rated life. DLR’s magnitude typically depends on screw diameter, screw lead, and the number of recirculating rolling elements (balls or rollers) inside the nut that simultaneously carry the load. DLR is also known referred to as C, and is typically provided by manufacturers for ball and roller nuts.

**Constant loads** remain unchanged along the full working cycle

**Varying loads** change during the working cycle. If an application has a varying load, it becomes necessary to calculate the equivalent dynamic load for life calculations.

**Equivalent dynamic load** **(or ****P**_{e}**)** is a dynamic load acting on the screw axially and centrally, which, if applied constantly, would have the same effect on screw life as all of the actual loads. And if the load is constant, then equivalent load = actual load.

**“****L _{10}**

**” Life**is the life of a ball or roller nut at which 10% of all its balls or rollers can be expected to suffer a classical fatigue failure (or alternatively, the life at which 90% will still be operating). Outside the USA, it may be referred to as “B10” life. L

_{10}for a group of identical screw actuators operating under the same conditions is the period within which 10% of them can be expected to fail. This means 90% of the actuators have the statistical probability of surviving that long.

L_{10 }life does not account for any other mode of failure such as lubrication starvation or incorrect mounting.

L_{10} is a theoretical life estimate based on statistics, so it is no guarantee of performance but a good predictor of expected life.

SH: Estimating Life with a Constant Load

To estimate life with a constant load, the underlying formula that defines this value is:

L_{10} = (C/P_{e})^{3} × l

L_{10} Travel life in millions of units (in or mm), where:

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

P_{e} = Equivalent load (lbf) or (N)

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

If the load is constant across all movements, then the equivalent load = actual load.

If the load varies across moves, then you must calculate an equivalent load.

Be sure to use the same units in both parts of the equation. Use the same units (lbf or N or kN) as DLR.

**Example** of calculating constant-load life

Given the following values for an application,

C = 10,000 lbf

P_{e} = 5,000 lbf

l = 5 mm

Calculate the travel life of the actuator.

The underlying formula defining this value is:

L_{10} = ((C)/P_{e})^{3 }× l

L_{10} = (10,000/5,000)^{3} × 5

= (2 )^{3 }× 5 = 8 × 5

= 40

= 40 million mm or (40,000 m)

### Estimating Life with a Varying Load

As explained earlier, when the load varies during the working cycle, it is necessary to calculate the equivalent dynamic load (P_{e}). Use the calculation below when the load is not constant throughout the entire stroke. In cases where there is only minor variation in loading, use greatest load for life calculations.

Source: Metalworkingnews