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Roller and Tire Lubrication

Why should the contact surfaces of the rollers and tires not be lubricated with oil or grease?

There is little chance that the meeting of the mating surfaces of the tire and the rollers through the pinch points is one of pure rolling action. There are several reasons for this. The most obvious is that the rolling axis of the roller is not parallel to that of the tire. It would be exceptional for this to prevail for any significant period of time given the nature of the equipment both in its design, installation and operation. Additionally there is always axial movement of the tire on the rollers caused by either or both thermal expansion and mechanical clearance between tire and shell and tire and thrust rollers. Then there is also tire wobble which is always present to some degree. Tire wobble at the pinch points is axial movement that cycles back and forth with each rotation. All of the above detracts from the surfaces meeting in pure rolling by introducing some amount of face dragging or skidding. Lubrication therefore is mandated to prevent surface galling as would be required for any similar materials sliding face to face. Reducing these extends the life of the components and can have a significant effect on the drive power requirements. In the past oil and grease have been used as lubricants but for a number of years, rotary vessel designers and suppliers and indeed end users have turned to graphite blocks as an alternative. The old grease/oil based lubricant can permeate the grain structure of the metal. As the rolling surfaces meet, tremendous hydraulic pressures are generated within the actual grain structure of the tire and roller. Liquids such as water and oil are incompressible and in compression, when going through the pinch point, are actually much stronger than steel.  Therefore as the liquid lubricant tries to escape it fractures the metal grain causing cracking and spalling of the running surfaces. Added to this is the affinity of liquid lubricants to attract dirt and debris this can result in impregnation damage to the rolling surfaces as well as the formation of a "grinding paste" which can quickly erode the contact surfaces and even support shafts if the "paste" is allowed to build up between the roller and bearing. Continuous application of liquid lubricants is also a problem both drip and pressure systems are prone to malfunction and a uniform film of lubricant is difficult to achieve which in turn makes control (float) of a vessel even more difficult. We would be the first to agree that proper lubrication is essential for maintenance of any mechanical device, tires and support rollers are no exception. Rotating vessel designers now almost universally agree that rolling surfaces should be lubricated with dry lubricants, such as graphite blocks. Properly selected graphite blocks not only provide excellent lubrication but also act as scrapers to prevent most dirt from rolling through the pinch point. They should be one-piece and at least as wide as the support roller. Use of one piece blocks will prevent product or dirt accumulation where multiple blocks join which can cause circumferential bands of pits and grooves on the rolling surfaces. Graphite blocks do wear but are easily monitored and replaced, the graphite debris is fine and dry and is easily cleared away, this is certainly not the case with grease or oil lubricants. We cannot readily recall any vessel suppliers who now advocate the use of oil, grease or even water as a lubricant on support roller faces. We trust the foregoing goes some way as to explain why we chose to supply graphite lubrication for your vessel, but if you have any queries or comments please do not hesitate to contact us.

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