Lubrication of Ball Screw Drives – Lubricants, Application Areas and General Information

Ball screws drives or recirculating ball spindles should never be used without lubrication, otherwise their functionality cannot be guaranteed and their service life will be shortened. Lubricants such as oil or grease may be used for lubrication. This article explains which lubricant is suitable for which reasons and how to optimally lubricate ball screws.

Ball Screws/Recirculating Ball Spindles

Ball screws, also known as recirculating ball spindles, are high-precision components that are primarily used wherever rotary movements need to be translated into and from linear movements. This is the case, for example, when lifting and lowering of fixtures in both horizontal and vertical directions. Precision is primarily caused by the low play of the ball screws. Thanks to this precision, ball screws can help generate a high degree of repeatability. If a pre-tensioned ball screws is used, the play can be even further minimized, which further increases precision. They work rapidly and reliably and are often used as drive for linear guides. It is recommended installing ball screws always with a fixed bearing or a floating bearing. This avoids vibrations and bending of the ball screw by compression.

Why do ball screws need to be lubricated?

Ball screws must be lubricated for smooth operation. The lubricant forms a protective film on all important parts of the ball screw (screw shaft, rolling surface of the nut and surface of the balls). Thanks to this lubricating film, friction is minimized, resulting in a more efficient operation, and preventing heat generation, which in turn is reducing the thermal stress on individual components. A good lubrication also contributes to increasing the already good precision of ball screws. Good lubrication also counteracts problems that inevitably arise when several metal parts come into contact, especially in moving components. Therefore, the lubrication of the ball screws is also a part of maintenance and it extends the service life of the ball screw.

Correct Application of the Lubricant

Ball screws must never be operated without lubrication, otherwise damages will occur. Ball screws supplied by MISUMI are already prefilled. In the event that a non-prefilled ball screw is used and initial lubrication is necessary at the time of commissioning or it must be relubricated, some instructions for correct handling are provided below.

The most important basic rule is: Always use the same lubricant. Different lubricants must not be mixed with each other. Because if mixed, undesired reactions between the lubricants may occur, resulting, for example, in clumping and other impurities, which can have a lasting negative impact on the functionality of the ball screw. Selecting the right lubricant depends on the area of use and the intended application.

Before the initial lubrication, rust-inhibiting oils and other substances must first be removed. It is relubricated within specified intervals. These intervals are as different as the applications themselves: They depend on the number of operating hours, operating conditions, or the application. Some operating conditions require more frequent lubrication. High operating temperatures, for example, can reduce the lubricity of a lubricant. High exposure to dust and other particles will lead to the lubricant getting dirty and replaceable more quickly.

For successful relubrication, the ball screw must first be checked for damages, foreign particles and contaminations. Unusual noises may also indicate a malfunction. In the next step, the ball screw must be cleaned with suitable cleaning agents and solvents. These agents must then be rinsed thoroughly off the ball screw. Now the function test is performed: The ball nut must be moved several times over the entire travel path over the dry, not lubricated spindle. If they run smoothly without resistance or without any noise, the roller body and spindle do not malfunction. The remaining lubricant dispensed by the roller bodies can now be removed with a dry, lint-free cloth. Then, the ball screw must be lubricated immediately according to the manufacturer's instructions and, if necessary, protected against rust and other damage using suitable agents. There are various methods to grease the spindle. If lubricated manually, the lubricant is applied with a brush or grease gun unit, for example. However, there are also automatic lubrication systems that provide precisely the right amount of lubricant.

Difference Between Oil and Grease Lubrication

Lubricating greases and oils provide protection against corrosion. However, they offer different advantages and disadvantages as well as possible applications.

Oil

Oil-lubricated ball screws may require less maintenance, as the oil removes contaminants and a large part of the possible contaminants as it flows through the ball screw nut. Unlike grease, oil is not pushed along by the threaded nut, whereby fewer deposits form.

Other benefits of oil are:

• Oil can be applied directly at the lubrication point. It removes moisture and dirt and can be used for cooling in addition to lubrication.
• Oils work well at low to medium operating speeds, temperatures and load sizes.

Under more extreme conditions, an oil coating can become unusable and the undetected metal-to-metal friction can rapidly cause damage. Overall, oils are more expensive than grease because they require a pump and a filter system for the application. Furthermore, they can be the reason for contamination on the ball screw if too much lubricant is used or lubricant is unsuitable for the application.

Greases

Greases can be applied directly to the spindle or into the ball nut if it has open bore holes that can pump the grease.

Other advantages of greases are:

• They are also suitable for high speeds of rotation.
• They are generally more cost-effective and often allow for long lubrication intervals.
• Lubricating greases can be processed with additives to synthetic lubricants that withstand extreme temperatures, loads and speeds of rotation.
• There are special vacuum or cleanroom quality greases.

It should be noted that some fats are not compatible and, for example, lead to clumping. Compatibility must always be checked beforehand. In addition, grease often accumulates with abrasive particles on the ball screw nuts.

For ball screws or ball recirculating screws, there are, for example, also specially developed spindle grease with special characteristics available. Spindle grease is highly viscous and ensures particularly low friction.

Additives

Additives improve the characteristics of fat. For example, greases can be processed with additives to synthetic lubricants that can withstand even extreme temperatures, loads and speeds of rotation. The addition of additives reduces wear and minimizes friction. Grease, for example, should not be used together with molybdenum disulfide or graphite, as the friction values they generate are too low.

Other benefits may include:

• Grease can better resist dirt
• Wear under load is reduced
• Vibrations are reduced

Various Grease Classes

The National Lubricating Grease Institute (NLGI) is an international trade association dedicated to promoting the development and research of lubrication technology. In addition, they publish industry standards for greases, e.g. the nine NLGI grease classes. These are standardized and recognized worldwide. The degrees range from very liquid to extremely solid. A penetration test at 25°C for the classification: The degree of grease can be determined based on the penetration depth that occurs when a standardized cone is pressed into the grease. The grease classes are suitable for various applications:

• NLGI 000, 00 and 0: at low temperatures, high speeds of rotation, in central lubrication systems
• NLGI 1 and 2: for general lubrication purposes, widely used, also suitable for ball screws
• NLGI 3 and 4: if a higher consistency is required (slow-rotating, heavily loaded bearing systems)
• NLGI 5 and 6: for special applications, e.g. in vertical and difficult-to-reach areas

Recommendations

To ensure that the lubrication is constant, the rule of thumb is to lubricate every six months under normal use and every three months under heavy use. This keeps friction and rolling resistance between the grooves and the ball bearings low. MISUMI ball screw products are usually filled with lubricant before delivery. Unless otherwise specified, a lithium-saponified lubricant or Multemp Grease PS2 is used as the standard lubricant depending on the product. As an alternative to standard lubricants, the following types of lubricant are available:

• L-Type Lubricant: RT-100K - excellent heat and oxidation resistance as well as adhesion and cohesion force. In addition, there are few splashes and low leakage.
• G-Type Lubricant: LG2 – cleanroom-compatible special lubricant for linear guides, ball screw drives, etc.

The following table provides an overview of the lubrication performance of lubricants used in ball screws and compares standard lubricants with lubricant types L and G:

Conditions at the Place of Use

In addition to lubrication, there are still a few conditions to consider when using ball screws at the place of use. Clean ambient conditions should prevail at the place of use, i.e. where dust or other particles could penetrate the ball screw covers should be used. Otherwise, the ball screw may be damaged and performance may be affected. The nut must not exceed the usable area (between the arrows) as otherwise the rolling elements installed into the nut can fall out and cause malfunctions or event total failure:

In addition, a ball screw should be used at temperatures of 0-80°C, otherwise damage to rotating parts or sealing parts may occur.

Excursus - Lubricants in Cleanroom Environments

Mechanical components are also installed in the cleanroom. In this environment, the number of particles present in the air are monitored and could potentially lead to contamination, as particularly sensitive components such as semiconductors are manufactured here. It is imperative to adhere to the limits. It is therefore not surprising that lubricants for ball screws are, for example, subject to particularly high requirements, if these are to be used in the cleanroom:

• The lubricant must not release any particles.
• It must be chemically stable so that it does not release reactive substances.
• At the same time, it must also be compatible with other materials in the cleanroom in order not to trigger a chemical reaction.

Synthetic lubricants are particularly suitable for the cleanroom, for example, because of their high purity. In addition, lubricants based on perfluorinated polyether oils and PTFE can also usually be used without any concerns. They offer good wear protection and are chemically resistant to sterilization processes in the cleanroom (e.g. solutions with hydrogen peroxide, use of hot steam).

At MISUMI, type G is ideal for use in cleanrooms, e.g. for linear guides and ball screws.