Design Engineer's Guide – Selecting a Lubricant for Ball Screws

Figure 2 - Oil flow rate guide. Required volume flow rate (Q_required) for a given application is typically the minimum rate called for (Q_min) adjusted for operating conditions. The flow rate is automatically adjusted for the number of circuits in the ball nut.

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%).

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;

Without the heavy load, application every 5 min would be adequate.

You can use 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 by replacing spot delivery with continuous flow of the lighter oil.

Selecting the Right Grease

Grease selection is a bit more complex than oil specification.  Greases consist of mineral or synthetic oil, additives, and a thickening agent such as lithium, bentonite, aluminum, and barium complexes. Some of the factors to consider are synthetic versus mineral-based, additives, and environmental conditions. Note that speed is not a criterion for selection since, unlike oil, grease can handle dn (bearing bore, mm x rpm) values as high as 1,000,000, which is about 10 times higher than dn values typically found in ball screw applications.

• Synthetic vs mineral based. Synthetic greases offer many performance advantages over mineral-based lubricants. They function over wider temperature ranges, offer greater stability and retain the viscosity needed for adequate film thickness through a specified range of operating temperatures, speeds, and loads.

• Additives. Special additives can improve the ability to resist contaminants and reduce wear in the presence of load and vibration. Other additives also reduce friction, decrease noise, and increase load capacity. Avoid greases with graphite or molybdenum disulfide, though, because they reduce friction so much that it promotes ball skid and interferes with the rolling process of the ball bearings.

• Specialty products. Specialty greases are also available including vacuum and food grade, as well as special formulations for clean rooms or extreme temperatures. Match the grease selection to the application environment and then select based on performance requirements.

To help engineers sort through the options, the National Lubricating Grease Institute (NLGI) has identified nine grades based on consistency. While not sufficient in and of itself to specify which type is right for a particular application, it does provide a useful qualitative measure. Figure 3 shows the nine NLGI grades, defined by a range of worked penetration test results.

Figure 3 - NLGI Grade

As shown, the lower numbers are softer and flow better, at the lowest extreme having viscosity similar to cooking oil. Higher numbers are firmer, tend to stay in place, and are a good choice when leakage is a concern. At the highest range viscosity is comparable to the consistency of cheddar cheese.

NLGI Grades 000 to 1 would typically be used in application requiring low viscous friction. Grades 0, 1 and 2 are used in highly loaded gearing while grades 1 through 4 are often used in rolling contact bearings, with Grade 2 being the most commonly applied viscosity in that range.

Where figure 3 helps determine which viscosity to use, figure 4 recommends how much grease to use in the nut per circuit based on nominal ball screw diameter.  A 25 mm
diameter ball screw with 6 circuits, for example, requires 0.4 cm³/circuit, or 2.4 cm³ total.

Figure 4 - Guide to grease quantity per circuit. To determine total volume required, you must multiply the volume determined from this figure by the number of circuits in the ball nut.

Based on the NGLI guidelines and information published by grease or ball screw manufacturers, figure 5 matches some of the leading grease brands to applications. While by no means a complete list, it should provide some useful guidance.

Conclusion and summary

The decision to lubricate with oil or grease can have a critical impact on performance, durability and total cost of owning and operating a ball screw. Running without lubrication is not an option.

In some cases the overall choice is dictated by other key specifications, but wherever possible, designers of ball screw applications should consider lubricant options as a factor in decisions to specify one vendor product over another. And once the decision to go oil or grease is made, there are many additional options.

Oil lubricants tend to be easier to control, run cleaner and better able to flush contaminants out the process, but they can also require more costly pumping and meeting support, and have a tendency to introduce new contaminants in to the system. Once oil is chosen, the challenge is to select the proper oil viscosity for an application, which can be calculated from the temperature, load, and speed by applying a standard equation.

Grease tends to be less expensive, has higher heat resistance and, has a wider range of consistency which can help accomplish more effective lubrication goals. Grease is more contamination free, but also has a tendency to pick up additional contamination as excess oozes from the nut. And, as with oil, once the choice is made, the next decision point is selecting the right type of grease for the application. The NGLI provides some guidelines to map consistency to applications.

Because options may vary with the vendor, especially for specialty applications it is always a good idea to get complete specifications from them, and it also critical to note that proper maintenance is critical to performance.