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 dnm 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:
nm = n1(q1/100) + n2(q2/100) + n3(q3/100) + ...
where:
nm = mean speed, rpm n1,2,3 ...
= speed for time q1,2,3 ..., rpm q1,2,3...
= time at speed n1,2,3 ..., % of total
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For typical applications, nm ranges from 200 to 500 rpm.
The dnm factor is given by:
dnm = (d)(nm)
where:
d = ball screw nominal diameter, mm
Typical values of dnm are in the range of 15,000 to 25,000 mm/min. Values of dn up to 100,000, where n is the maximum speed of rotation, are becoming more common, and in such cases the lower viscosity should be used if the oil selection guide indicates a grade midway between two adjacent viscosity curves, such as VG32 and VG46.
Ball nut operating temperature should be about 20 C, but this is seldom the case in real operation. Usually, a ball nut stabilizes a few degrees above screw-shaft operating temperature. If you can’t measure nut temperature, assume it to be 30 C for your initial selection of oil viscosity.
Required oil flow rate is a function of:
• Number of ball circuits.
• Ball-screw orientation.
• Operating environment.
• Load.
• Speed.
• Judgments based on knowledge of the application.
The oil flow rate guide of Figure 2 helps determine oil application requirements. It shows a range of flow rates in ml/hr for various values of the product of ball-screw nominal diameter (mm) and number of ball-nut circuits. For example, for a ball nut with 6 circuits and 25-mm nominal diameter, the product, 150 mm, shows a flow rate ranging from a minimum (Qmin) of 13 ml/hr to a maximum (Qmax) of 26 ml/hr.
The rate you should select within this range depends on operating conditions:
• If the ball screw is horizontal, add nothing to Qmin to account for orientation; if vertical, add 25%.
• If the application is clean and dry, add nothing to Qmin to account for environment; if not, add 25%.
• If the screw is not subject to high loads or speeds, add nothing to Qmin to account for severe running conditions; if it is, add 50%.
For example, the 25-mm diameter ball screw with 6-circuit ball nut is used in a vertical application (add 25% to the minimum of 13 ml/hr); it is clean and dry (add 0); and it is heavily loaded (add another 50%).
Qrequired = 13 + (13 × 25%) 10 + (133 × 50%)
Qrequired = 13(1.75) = 22.75 ml/hr
Because it is best to supply a small amount of oil at regular intervals and you anticipate heavy loads in this example, apply oil about every minute:
(22.75 ml/hr)(1 hr/60 min)
= 0.38 ml/min
Without the heavy load, application every 5 min would be adequate.
Experience has shown that you can use a slightly thinner oil than indicated without difficulty. However, if the application calls for, say, a VG46 oil, but a VG10 is all that can be used, you would have to revise the oil delivery system to replace spot delivery with continuous flow of the lighter oil.
Grease lubrication
Grease is not so widely used as oil for ball-nut lubrication, though it lubricates acceptably. Speeds that are high for ball screws are no problem for grease, so speed is no criterion for selection. For example, grease is the accepted lubricant for machine-tool spindles with dn (bearing bore, mm × rpm) values as high as 1,000,000. For ball screws, dn values rarely exceed 100,000.
One problem with grease: It tends to be fed out of the nut and onto the ball screws, accumulating at the extremities of travel where it collects contaminants. Thus, you must replenish it regularly.
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Grease is a complex subject. Greases consist of a mineral or synthetic oil, additives, and a thickening agent such as lithium, bentonite, aluminum, and barium complexes. For most applications Thomson Industries recommends a grease with a drop point above 220 C, a service temperature range of -30 to 130 C, and a limiting speed factor (dn) above 1,000,000. Such a grease is classified as HL91 Grade 2 (DIN 51818), and is based on Mil-9-7711A.
The guide to grease quantity per circuit, Figure 3, shows the recommended quantity in cm3 per ball circuit in the nut as a function of nominal ball-screw diameter. For example, a 25-mm-diameter ball screw with 6 circuits requires 0.4 cm3/circuit, or 2.4 cm3 of grease total.
As a rule of thumb, replenish grease at least every 800 hr. However, because conditions vary so widely, you should confirm this interval by inspection, and readjust if needed. For extreme conditions, such as dn values above 50,000, consult a lubrication expert.
The right oil or grease lubrication for ball-screw assemblies reduces unscheduled downtime for repair by ensuring that the assemblies deliver their expected service lives.
For more about ball-screw assemblies by Thomson Industries Inc., Port Washington, N.Y., circle 403 on the reader service card.
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