Lubricants maintain the low friction advantage of ball-screw assemblies by minimizing rolling resistance between balls and tracks and sliding friction between adjacent balls. Proper lubrication helps keep most contaminants out, which greatly reduces the damage foreign matter can cause.

There are several methods of delivering lubricant to a ball-screw assembly, and several methods to retain the lubricant in the lubricated assembly. For example, a grease-lubricated ball nut may have a grease fitting on the nut flange (if there is a flange) or on the nut body. And the fitting may be oriented radially or axially. The purpose of this article is to discuss the lubricants themselves, not the means of delivery.

Lubricants are often taken for granted, but the right choice for each application ensures a ball screw that performs properly for its expected life. Both oil and grease provide corrosion protection, but lubricant choice depends on evaluation of the advantages and disadvantages of each in the given application.

You can apply oil at a controlled flow rate directly to the point of need, and it will clean out moisture and other contaminants as it runs through the ball nut. It can also provide cooling. Oil disadvantages you must consider include:
• Possibility of excess oil contaminating the process, such as mixing with the cutting fluid in a machining application.
• Cost of a pump and metering system to apply oil properly.

Grease is less expensive than oil to apply and requires less frequent application, and it does not contaminate process fluids. On the other hand:
• Grease is hard to keep inside the ball nut and has a tendency to build up at the ends of ball nut travel, where it accumulates chips and abrasive particles.
• Incompatibility of old grease with relubrication grease can create a problem. Be sure to check compatibility.

Oil lubrication

Operating temperature, load, and speed determine the oil viscosity and application rate needed for each installation. If the oil is too viscous or if you use too much, heat may be generated. If the oil is too thin or you use too little, parts may not be coated adequately; friction and wear may result.

The following guidelines are appropriate for most applications, but if extremes of temperature, load, or speed are involved, you should consult a lubrication specialist. We use metric measurements in these guidelines in accordance with current needs of designers, especially those in the machine-tool industry. If inch-size ball screws are used, conversion to metric dimensions is all that is needed, although metric ball screws are now specified at the outset in most cases.

The recommended nominal viscosity of the oil at 40 C is based on the mean speed of the ball screw, its diameter, and the temperature at which the ball nut is likely to stabilize. Viscosity is expressed in centistokes. (1 cSt = 1 mm²/sec.) Various grades have been selected for standardization (DIN 51512) and are used in the oil selection guide of Figure 1. For example, VG32 is an oil with nominal viscosity of 32 mm²/sec at 40 C.

To determine the nominal viscosity of the oil for an application, you need to establish the mean speed of rotation of the ball screw and, from it, the dn_m factor. You also need the temperature at which the ball nut is likely to stabilize. Mean speed of rotation accounts for the ball screw’s duty cycle:

n_m = n₁(q₁/100) + n₂(q₂/100) + n₃(q₃/100) + ...

where:
n_m = mean speed, rpm n_1,2,3 ...
= speed for time q_1,2,3 ..., rpm q_1,2,3...
= time at speed n_1,2,3 ..., % of total

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