Spindle bearings must be properly lubricated to maintain a film of lubricant between the rolling elements and the bearing raceways. Without lubrication, the rolling elements will have direct contact with the raceways, and this will result in heat, race deterioration, rolling element deterioration, cage disintegration, and ultimately bearing/spindle failure.
There are several methods of lubrication used for spindles. Below you will see the different types and the advantages and disadvantages to each method of machine tool spindle bearing lubrication:
Grease packed spindles are some of the most commonly used spindles in machine tools today.
The majority of spindles are greased for life, meaning the only grease needed is that which is installed at the time of assembly. Grease manufacturers have brought their formulas to the point that the lubricating oils and the agents that carry them are resilient enough to last longer than the bearings expected duty cycle. However, bearing life can still be dramatically decreased by the introduction of outside influences such as coolant, high heat, metal chips, and grinding swarf—to name a few.
Spindles that are not greased for life and require additional grease to be added are uncommon and typically low speed.
Grease injected spindles allow for a small portion of grease to be injected into the bearings at a rate determined by the machine tool manufacturer. This is commonly done with grease cartridges that are pneumatically controlled by the machine PLC. This allows for new grease to replace what is currently in the bearing and the ability to use different grease types in a spindle that may have a combination of angular contact bearings and roller bearings.
Oil lubrication is performed by external equipment and usually carried to the spindle via tubing. These systems use liquid oil, such as Mobil DTE light or equivalent, that is delivered to the spindle in a stream of air. There are a few different delivery systems, each having its own characteristics:
One of the first spindle oil delivery systems used for spindles was oil mist. This method uses compressed air to atomize the oil and carry it to the spindle. This oil laden air is pumped through the spindle flowing through the bearings. Some of the oil “wets out” on the bearings while some of it is released to the atmosphere from the front and rear of the spindle. The system consists of an air regulator attached to a ventury. The ventury often has a clear bubble at the top with a tube that shows the frequency of the drops per minute. For spindles, an average rate is 20 drops per minute (dpm) but can vary from 5 dpm to 80 dpm depending on the application. Too high of a rate causes heat and bearing skidding and too low a rate does not allow for enough of an oil film between the rolling elements and raceways.
Oil injection systems are comprised of three main parts: Air filter/regulator, oil pump, and injector block. Most manufacturers (we recommend Bijur) have a panel with the entire system installed on it. The pump contains the oil reservoir and is set to activate at predetermined intervals. It produces enough pressure to activate the injector cartridges in the mixing block. The mixing block produces a finely metered drop of oil, usually between .01 and .05 cc’s each time the pump activates. The oil drop is then pushed down the wall of the tubing to the bearings by a steady stream of air. This system requires the oil lines be run very close to the bearings, unlike oil mist systems.
This method lubricates the bearings through holes in the inner races of the bearing. Oil is pumped through the shaft, through the bearing, and then recovered by a vacuum system.
Hydrostatic bearings and hydrodynamic bearings are fluid film bearings that rely on a film of oil to create a clearance between the moving and stationary elements.
Hydrostatic bearings employ a positive pressure supply that maintains clearance between the rotating stationary elements. With a hydrostatically lubricated bearing, the lubrication is introduced under pressure between the moving surfaces. Hydrostatic bearing spindles feature high stiffness and long bearing life, and are often used for fine machining and finishing. Because hydrostatic lubrication does not depend on relative motion to maintain the lubrication film, it can accommodate heavy loads at low speeds.
A hydrodynamic bearing is typically a low-clearance assembly that relies on a film of oil that develops clearance while the spindle is rotating.
To learn more about machine tool spindle bearing lubrication and how to properly lubricate your spindle bearings and maintain a healthy film lubricant between the rolling elements and raceways, contact Northland Tool & Electronics. Beyond spindle repair, we provide full-scale spindle support services. Our spindle support program includes:
We also provide comprehensive reports on what we have observed along with recommended corrective actions. Contact us today to get a quote for your spindle repair job or check out our case studies to see why we consider ourselves among the best in the spindle repair business.
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