Like all motion control devices based on contact friction, ball screws require lubrication for proper functionality. The reason is that lubrication is required for smooth recirculation of the rolling elements. Even if a dry lubricant coating is applied to the ball screw elements (shaft, nut and balls), some small amount of conventional oil/grease is still required. In other words, the ball screw cannot properly operate while completely dry. All Steinmeyer ball screws are pre-lubricated at the factory – but only with an initial fill amount. For most applications, we recommend an initial lube check after 500 hours of machine time. Based on that inspection, adjust the maintenance schedule accordingly. Of course, every application is different, and users should develop their own maintenance schedule based on the particulars of their operation.
One common scenario that can cause lubrication challenges is repeated use of a small segment of the total working volume. For example, let’s say your X and Y axes have a total travel of 1 meter, but you are machining an area of the part only a few centimeters long. If your machine performs this task repeatedly on many parts, a large portion of the X and Y shafts will be exposed to the environment without ever being swept by the nuts. Plus the balls inside the nut will not receive fresh lubricant. As a result, they may lose lubricant while gaining buildup of dirt, dust, and swarf. These conditions increase the risk of excessive wear when the machine returns to other operations.
To guard against this scenario, we recommend two solutions. The first is to create a designated lube “station” at the opposite end of the small operating range. Move the nut to this “station”, apply lubricant, and then return. This operation will ensure the shafts are not only wiped of debris (presuming there are proper wipers at both ends of the ball nut) but also evenly lubricated. Alternatively, if shaft contamination is minimal, you may inject lubricant into the nut while still in the small operating range. Move the nut at least one length to distribute the lubricant evenly within the nut. When the machine returns to other operations, there will be sufficient lubricant inside the nut to cover the shafts properly. See the Steinmeyer web site for re-lubrication guidelines for both oil and grease – lubrication quantities vary based on shaft diameter and nut type. Another alternate solution is to lubricate through the shaft – that is the shaft is cross drilled to allow lubricant to pass through it and into the ball nut at a designated location. This is especially useful for certain oscillating motion situations. For any further questions or suggestions please contact one of our application engineers!
Customers often ask us: what type of lubricant is best for my ball screw application? And which is better, oil or grease? The truth is that both oil and grease can perform well in a variety of situations.
If the duty cycle is such that an elastohydrodynamic (EHD) film can build over a significant part of the motion, then oil lubrication (with a properly selected grade and viscosity) will always outperform grease in terms of wear. Only oils with wear inhibiting additives should be used.* These have the ability to lubricate in conditions of boundary friction, when speeds for EHD-lubrication are insufficient
On the other hand, grease has an edge at slow speeds because it offers better wear protection under mixed friction or boundary friction conditions. Also, lubricant loss with grease is lower than with oil, so grease can be used for long-term or “for life” lubrication. Attention must also be given to the wiper choice in all cases.
Steinmeyer has extensively studied the performance of a wide variety of oils and greases. Some of this work was conducted by the tribology lab of a major European research center. Details are available on our web site. This test set-up was designed to mimic the specific tribological conditions found in industrial ball screws, both in terms of lubrication film build-up and wear.
One key finding was that the coefficient of friction does not seem to be correlated to the wear rate. Some lubricants yielded low friction, but higher wear at the same time! With the exception of a high-pressure grease, which caused the lube film to collapse at high speeds, all greases performed well and yielded acceptable wear rates throughout the test scenario. At the same time it was proven that greases are unable to build a perfect fluid film like oils of proper viscosity, so the wear rate with grease lubrication is higher than the wear rate of oil when an EHD film is present.
So oil and grease both have different advantages. The final choice will depend greatly on your application and desired maintenance schedule. Please contact Steinmeyer for a detailed consultation with our engineers!
* We strongly recommend CLP grade gear oil per DIN 51517-3 or equivalent. Do not use way oils or hydraulic oils, even if they are labeled “high pressure”! Look for the words “suitable for gear boxes”.
Proper functioning wipers are an essential element of any ball screw system. Their first job, of course, is to shield the contact surfaces from contamination. But some wipers may also serve as a lubricant reservoir, thereby lengthening service intervals. Fortunately, Steinmeyer offers the industry’s widest choice in wipers:
• Classic labyrinth seal
• Felt ring
• Combination (friction and felt)
Combination wipers are plastic wipers with an additional felt wiper inside the ball nut. Felt is an excellent absorber of oil, so it acts as a reservoir. However, it must be protected from water or water based fluids. That is one function of the additional plastic wiper.
Combination wipers increase the nut length slightly, but offer numerous important advantages:
• Superior dual stage sealing
• Reduced maintenance
• Reduced oil consumption (75-90%)
• Choice of lubricant
These last two points are especially notable compared to oil-impregnated plastics, used as wipers by some of our competitors. These materials are often claimed to be maintenance-free. But the truth is that the oil eventually wicks out. With felt, you can easily refill during normal maintenance intervals. Plus, the choice of lubricant is yours.
At Steinmeyer, we are incorporating combination wipers more and more into our designs.
Hah – got your interest with that title, right? But is there really such as component as a ball screw that requires zero maintenance? First we should establish exactly what we mean by “zero” maintenance. This can be one of two scenarios: either the ball screw has absolutely no lubrication, that is it runs dry; or the ball nut includes enough lubricant and is sufficiently protected such that no further maintenance is required for its lifetime. BTW for either scenario the answer is pretty much “NO, it’s not possible!” Continue reading
A question that often comes up for miniature ball screws – how do I lubricate it if there is no port on the ball nut? Good question! If you check our web site there is quite a lot of detailed information on lubricant types and wiper types, including precise amounts to use when re-lubricating:
There is certainly a wealth of information on lubrication on our web site – here is a link to the main page regarding oils and greases: http://www.steinmeyer.com/en/technology/drive-technology/lubrication-and-wipers/
There are however two issues that should be emphasized: Continue reading