Running dry

Lubrication reduces friction, minimizes heat generation, and retards wear and corrosion. Lack of it is the most common cause of ballscrew problems. In fact, operating without oil or grease can almost immediately destroy the screw.

The shaft should be kept damp with the lubricant that best matches the application. Each type of grease and oil reacts differently to temperature and other environmental conditions based on their additives. For most applications, such as those with operating temperatures from 0 to 180° F, you can use a good grade mineral oil for rolling element bearings. For those applications requiring grease, use a good NLGI Grade 2 bearing grease with EP additives. Other types of greases and oils are available for temperatures above or below this range. The ballscrew manufacturer can guide your selection.

The type of lubricant selected affects the application frequency. In addition, many lubricants indicate if the frequency must change. For example, if your lubricant is changing color or evaporating, then apply it more often.

The orientation of the ballscrew will also affect lubrication. For example, on vertically mounted screws, use methods to ensure the lubricant stays evenly distributed.

Lack of support

Both fixed and supported ballscrews need adequate bearing support because speed and length affect how the screw reacts to the load. To prevent damage to either the screw or the ball bearings, size the support bearings according to the manufacturer specifications. Be aware, though, that problems can occur through stress risers at journal diameters. So use generous radii and correct bearing fits.

Bearing systems should reduce stress concentrations created by pulleys, gears, and belts. These drive systems tend to bend the end of the screw shaft, fatiguing it back and forth as it rotates. If not compensated, the stresses will eventually ruin the screw.

A shocking experience

Sudden load changes happen in manufacturing, but such impact and shock loads are detrimental to the ballscrew ball bearings. They create forces that can cause the balls to run roughly, bring about race brinneling, or even fracture the balls. If shocks may occur in the application, be sure to provide adequate dampening.

The long and short of it

There's another load that if ignored can damage a ballscrew. It's known as the maximum compression load and is the ability of a ball screw to avoid buckling under this load. It is a function of the square of the length of the screw, it is usually much lower than the screw material's compression strength. Thus, the longer the shaft, the more prone it is to buckling, so the screw's column strength is often the controlling design parameter.

In addition to being a function of length, compressive load strength is also a function of the cross-sectional moment of inertia and end-fixing. Follow the manufacturer's guidelines when specifying the length of the ballscrew or put long columns in tension.

Get it straight

Attention to detail is crucial to proper ballscrew mounting. Even if the installation is off by "just a hair," it will shorten ballscrew life. Any misalignment among the ball nut, screw, bearings, and mounting surfaces can create internal forces that increase wear or cause excessive heat or vibration.

To properly align a screw, first, always be sure to support it on its outside diameter. The ball nut should never support the weight of the assembly. Remember: you need to react loads through a pure axial thrust direction.

Next, assemble the support bearing to the ballscrew and place this assembly on the mounting bed. Fasten the bearing support at the drive end to the bed, but only hand tighten the bolts. Then fasten the nut to the carriage or cross-slide and hand tighten those bolts.

Manually rotate the ballscrew, moving the carriage to the drive end. Now fasten the nut securely, tightening bolts to the manufacturer's recommended torque.

Too much to bear

A good way to damage a ballscrew is to make it move an excessively heavy load. How much is too much? To determine that, you need to consider two factors. Dynamic capacity is the rated axial load that can be applied during linear travel for a given life expectancy. Static capacity is the maximum axial load. Most manufacturers' catalogs offer load ratings or calculation information.

In some cases, you can exceed dynamic capacity. But the ballscrew will probably have a shorter life because the extra load may accelerate fatigue or the plastic deformation of the balls and raceways.

If you exceed static capacity, the load will permanently deform the raceway track. If ballscrew operation is rough and noisy, this may be the reason.

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