**STATIC LOAD**

Static load rating is the maximum non-operating load capacity. In all application the total weight of the load should not exceed the static load capacity of the ballscrew.

If a ballscrew is subject to a tension load, the static load capacity of the screw should be verified. However, if the screw is subject to a compression load, one that tends to buckle the screw, both the static and compression load capacities of the screw must be verified.

**APPLIED LOAD**

The applied load (the load seen by the screw) is dependent on whether the screw is to be used in a horizontal or vertical application, the total weight of the load that has to be moved, and the number of ballscrews used to move the load.

Whenever possible, the load direction should be coaxial with the screw. Overturning or cocking type loads should be avoided.

In vertical applications, such as lifting or jacking, the applied load is equal to the total weight of the load that has to be moved.

In horizontal applications the load seen by the screw is equal to the total weight of the load times the coefficient of friction for the type of ways used to guide the load as it moves.

Most applications only require one ballscrew. However, if more than one screw is required, the load seen by each screw is dependent on the distribution of the weight between the screws. For example, if the weight is equally distributed among four screws, the load seen by each screw would be one-fourth the total load.

**DESIGN LIFE OBJECTIVES**

After determining the applied operating load, the next step is to determine the design life objective, measured in inches of travel, for the application.

For a horizontal application, the formula for calculating the design life objective is:

*Design Life = S x C x H x D x Y*

Where:

- S = Stroke Length, in inches
- C = Cycles per hour
- H = Operating hours per day
- D = Working days per year
- Y = Expected design life for the application, in years For a vertical application, the above formula must be multiplied by 2 because the load is always applied to the same side of the ball groove.

**LOAD/LIFE RELATIONSHIP**

The load-life relationship for a particular size ballscrew is an inverse cube ratio, analogous to the B10 rating common to the ball bearing industry.

For example, reducing the load on the ballscrew by 1/2 increases the life expectancy 8 times. Doubling the load decreases life to 1/8 the original expectation.

This makes the life of ballscrew predictable based on the applied operating load that has to be moved and the total inches of travel defined by the design life objective.

In most ballscrew applications, where the load is in tension, selection of the proper size ballscrew can be made based on the load-life relationship for a stated load and design life objective. However, in those applications where column loading, critical speed, or other factors come into play, additional considerations must be made.

**CRITICAL SPEED**

Critical speed is the speed at which the nut or screw has a tendency to develop severe vibrations. Under normal operating conditions, the maximum safe operating speed of a ballscrew assembly is 80 percent of the critical speed rating for the screw.

Critical speed is a function of the unsupported length of the screw, the screw diameter, and the type of end bearing supports. The unsupported length is the dominate factor, because the critical speed is inversely proportionate to the square of the unsupported length.

**COMPRESSION LOADING**

Compression or column loads have a tendency to cause a ballscrew to buckle. Therefore, whenever a ballscrew is subject to these types of loads, the safe compression rating of the ballscrew must be verified.

The compression load capacity is dependent on the unsupported length of the ballscrew, the type of end bearing fixity, and the ballscrew diameter.

Need help? Let Comptrol size, design and manufacture your next screw application. Contact us at 216-587-5200 x2