Moulded oil seal features simultaneous sealing and lubrication
Original article date: November 1997
NSK’s K1 Seal for ball screws and linear guides will enable machine builders to make real cost savings by replacing existing, and often costly, methods of lubricating precision machine parts.
Ballscrews and linear guides are essential components in any precision machine. Both have undergone remarkable developments over the last decade and these have contributed to the astounding speed and accuracy improvements which have been achieved in production machines.
The effective and reliable operation of ball screws and linear guides depends upon a number of factors, not least effective lubrication and sealing against the ingress of contaminants. These factors are particularly critical on machine tools, woodworking machines and in materials handling applications.
Depending upon the application, either grease or oil lubrication is used with ball screws and guides. In the case of grease, this entails a planned lubrication regime for the components, or, with oil, the integration into the machine design of a costly forced lubrication system.
Recently, increasing attention has been paid to plastic composite materials containing abundant lubricating oil (or grease) as a replacement for conventional lubricants. NSK has developed a new material called “Molded Oil” which is a plastic containing lubricating oil. Differing from other marketed oil-impregnated plastics, where the oil content is a few percent by weight, Molded Oil permits a lubricating oil content of more than 50% by weight.
The oil discharge rate from the new material depends on the operating temperature. Molded Oil has lubricating oil enclosed among polyolefin molecules. When the temperature rises, the polyolefin molecules tend to shrink, resulting in discharge of the lubricating oil. This means that more lubricating oil is supplied through slide heat generation when needed.
The K1 Seal was developed so that linear guides and ball screws could take advantage of Molded Oil. The new seals play a double role in that they achieve the functions of sealing and lubrication simultaneously.
The major features of K1 Seals are:
* Extended lubricant life, outperforming forced oil lubrication.
* Consistent lubrication is maintained and this extends the maintenance free period, even under heavy loads and in the presence of contaminants.
* Environmental and maintenance costs related to the supply and disposal of forced oil lubricants are eliminated.
In tests, linear guides equipped with K1 seals achieved lubrication-free running to distances in excess of 25,000km. The K1 seals also performed well in durability load tests, achieving twice the service life of the standard double seal at an operating speed of 24m/min.
The K1 seals are particularly effective in environments where oil related pollution must be prevented. The food industry with its strict hygiene regulations is a good example, as are semi-conductor and medical equipment manufacture.
Another problem which the K1 Seal effectively addresses is loss of lubricant due to washdowns. Machines used in food production, pharmaceuticals and construction would normally suffer in this respect.
Other areas where the K1 seals make a real difference include environments where lubrication is difficult to apply, and those – woodworking for example – where oil- and grease-absorbing dust is produced. In both cases long term linear guide and ball screw performance is greatly increased by the efficient oil lubrication the seal provides.
Two examples are the new Hitachi Seiki HS 630 and HS 500 High Performance Horizontal Machining Centres. Both operate at higher speeds and under more severe conditions, taking higher loads than standard machining centres. The HS 630 and HS 500 machines must maintain micron levels of accuracy and high rigidity. As such, the K1 went through numerous design phases and exhaustive trials where it was proven to outperform traditional intermittent forced oil lubrication.
On the Hitachi Seiki machines, the K1 Seals on the ball screw nut and linear guide slider are packed with grease. The lubricant in the K1 gradually seeps out onto the rolling surface to counteract grease loss and deterioration. Re-greasing is not required as the grease itself is a long-life type making the unit maintenance-free. These properties have been tested under severe environmental conditions and in the presence of contaminants.
Ttests were conducted with cast iron powder mixed into the cutting fluid. With only grease lubrication, friction caused significant damage. But when fitted with a K1 Seal, pre-load was maintained for six times longer with no deterioration, representing a ten-fold increase in service life. Compared with tests conducted using intermittent forced oil lubrication, only half the friction occurred. The tests demonstrated that grease with the K1 Seal is clearly superior to the existing technology.
1 A new oil-impregnated seal ensures simultaneous sealing and lubrication of linear guides and ballscrews.
2 Hitachi Seiki is using Moulded Oil K1 seals on its ECO-ECO machining centres
November 1997