The first part of the mill that we will look at is the TRUNNION BEARING.This is a HYDROSTATIC BEARING which is a slow moving bearing that carries a heavy load. Usually it is constructed from Babbitts metal. If you happen to remember the introduction to this course Issac Babbitt was the man who devised the method of bonding self-lubricating metals onto a strong backing to form a bearing. This type of bearing is named after him. Let us start with the base of this bearing. In it you will notice a SUMP for oil and an oil line that is connected to a PUMP. In the centre of the base is the BEARING SEAT. There is a hole in the middle of the bearing seat to allow the oil from the oil line to come out of. Radiating away from this hole there are OIL CHANNELS. These are simply grooves cut into the metal to allow the oil to be dispersed over the face of the bearing.
Now this is important, as I mentioned, this is a hydrostatic bearing, it supports a great weight. Because it has a great weight on it, when it is stopped, the oil is squeezed out from between the inner and outer bearing. Before the mill is started up this bearing must have oil injected into it by way of a pump. The pump must be started before the mill begins to turn and must continue until the mill has made at least one revolution. After that one revolution, what is known as the wedge effect, takes place. The oil that is on the surface of the bearing is forced into a wedge shaped portion of the leading edge of the outer bearing. This will generate enough pressure to lift the entire mill. But if the pump isnt kept pumping for that revolution the weight of the mill as it turns will force the metal of the bearing surface into the oil channels. Two of the required characteristics of these bearings are that they resist dirt contamination and disperse heat well. The result is that the material that the bearing is constructed from is soft. If an oil film isnt present between the two bearing surfaces, the metal to metal contact will cause this damage to the oil channels. Then the bearing will not be able to get enough oil on start up. In extreme cases the bearing may be destroyed.
Once the mill is turning the bearing is self-lubricated by OIL RINGS. They should be checked periodically. If you look closely at them you will notice that they have holes in them. As the rings revolve with the bearing these holes pickup oil from the SUMP in the bearing base and carries it to the top of the bearing. This ring is not joined to the bearing but sits on it loosely.
Because of the drag on the ring by the oil in the sump as the ring is revolved through it, and the lubrication of the oil between the ring and the bearing, the ring turns at a slower rate than the mill does. This is necessary to allow the oil to flow out from under the ring and get caught at the contact point of the inner bearing and the outer bearing. The outer bearing then picks up this oil and maintains the wedge effect.
This cap provides access to the rings and the inner bearing by two ACCESS DOORS on top of the cap. When the bearing and rings are checked through these ports, the person doing the inspecting will be looking at the condition of the bearing surface, verifying that the bearing is being oiled, and that it is free from any grooves or gouges. At the same time, check the rings to be sure they are turning, but at a slower rate than the bearing is.
There is one more piece to complete the assembly, technically it belongs with a following section on liner description, but to avoid confusion I will include it in this portion. It is the TRUNNION LINER, it fits inside the bearing to protect it from wear caused by the ore being washed over the liner and through the bearing as it is fed to or discharged from the mill.
Swivel type lead-bronze bushed trunnion bearings are generally furnished on large diameterball Mills. The bearing swivels are of Meehanite metal spherically turned outside and bored and faced inside to receive the removable bushing. The bushing is bored and scraped to fit the mill trunnion. The bushing is provided with end flanges thus assuring that the trunnion flanges run against a bronze face.
On smaller mills rigid or swivel type bearings support the mill trunnions. The lower half of such bearings are lined with bronze or a special millbabbitt which is peened in place and bored to fit the trunnion.
In all cases a low bearing pressure is maintained to assure freedom from overheating, long life and minimum maintenance. They are designed to provide support to the mill proper, its media and pulp load.
In many dry grinding applications, or where heat is developed, the trunnion bearings can be furnished for water cooling. This system carries away the excess heat transmitted through bearings and protects them.
Lead Bronze has been found to be the most satisfactory bearing material for large diameter bearings, affording the greatest protection against damage of trunnions. Lead bronze will withstand extreme heat for a considerable period of time (for example in the event of lubrication failure). Such heat will cause the lead to sweat out and act as a lubricant itself. This protection eliminates the possibility of scoring a trunnion and there is no danger, as with babbitt, of having the trunnions settle in the bearing and rub on the bearing base.
These are cast heavy in section of Meehanite metal. Where swivel type bearings are used the base is spherically bored inside to gauge to receive such swivels. The bottom of the base is planed to fit a planed top of the trunnion bearing sole plate. The bearing cap is provided with a shroud feature extending out over drip flanges to protect the bearing from entrance of dirt or grit.
Slotted holes are provided in the base for bolting the base to the sole plate; this permits movement of the bearing on the sole plate for adjustment of gear and pinion mesh. Such adjustment is carried out by the use of set screws.
A separate hand operated lubricant jack can be furnished to be mounted on the bearing base or at some distant point to provide a flow of lubricant prior to starting mill rotation. This feature assures lubricant being present at the bottom of the bearing and reflects somewhat in reducing bearing wear and shows a slight reduction in starting torque.
Pinion shaft bearings are of the SKF anti-friction type mounted in a common twin bearing assembly. Bearings are fixed in place so that the pinion shaft of the mill is always in alignment with the drive components. V-belt driven mills are furnished with an outboard bearing of similar construction.
Also available are bearings of the double rigid ring oiling type for special applications. Such bearings are cast integral with a heavy twin bearing sole plate assuring perfect alignment and rigidity. These bearings are equipped with bronze or babbitted bushings.
There is a vast range of linear motion products on the market and it can be difficult to know which product to select. This article discusses the various types of products available according to their rolling element and explores their advantages, disadvantages and limitations. Finally a selection chart clearly lists each products characteristics to help with specifying the best linear motion product.
The primary purpose of linear motion products is to move something from one place to another in one direction along a straight line. Usually this is achieved using a bearing with a rolling element. Traditionally this rolling element would be a ball or roller, but these days some linear motion bearings do not incorporate a rolling element and consequently are known as plain bearings. These plain bearings are simply plastic or ceramic bushes and are sometimes lined with PTFE or oil impregnated plastic to help reduce the sliding friction.
It is possible to achieve linear motion without bearings but moving a load without a bearing requires a lot more effort. Using a bearing significantly reduces the friction. For example the coefficient of friction for a recirculating ball bushing can be as low as 0.001 whereas steel against steel (with lubrication) has a coefficient of friction of 0.10.
Roller bearings have a higher basic load rating than equivalent sized ball bearings. However they are less tolerant of lateral forces. Roller bearings are predominantly used as track rollers but they can also be used in cross roller guides.
Plain bearings (or bushes) have evolved rapidly over the last 20 years with the development of low friction plastics. Plain bearings are typically made from plastic or ceramic. Whilst they offer reduced friction, usually the coefficient of friction is considerably higher than rolling element bearings.
When selecting linear motion products there are many factors to consider. The range of products available is vast and each product comes in many shapes and sizes. Obviously price is always a factor, but each system has its own advantages and disadvantages and these should be taken into account when designing.
When specifying linear motion products, it is important to consider all the pros and cons of each product. Special attention should be paid to the friction as the coefficient of sliding friction (plain bushes) is markedly higher than that of rolling friction (balls & rollers). The lowest value of friction in the table above is 0.001 whilst the highest is 0.4, a factor of 400 times higher. Clearly the force required to drive the system is considerably higher for plain bushes (and in particular plastic bushes) but that said, they can still serve a purpose in lightweight applications.
For high accuracy applications recirculating ball bushings and ball guides are still the industry leaders. Ball bushings are versatile as they can be used fully supported (open type) or end supported (closed type). Linear ball and roller guides must be fully supported from either above or below. Whilst the roller linear guides offer higher load capacities, recent product development of the ball guides has narrowed the gap between their load capacities. Ball guides continue to be more popular than roller linear guides and consequently the prices tend to be lower.
Where dirt is an issue, the track bearings are clearly the best solution. They offer low coefficients of friction and there are different systems to suit a wide range of loads (e.g. cam followers, V-shaped bearings, combined roller bearings).
The miniature market tends to be fulfilled by recirculating ball bushings & miniature ball guides. Due to their extremely precise nature they are expensive and should only be specified when absolutely necessary.
Corrosion resistance can be achieved with both stainless steel products and specialist coatings. The range of products is limited and the costs can be substantially higher than the standard products. Again, only specify corrosion resistance where it is essential (e.g. food, pharmaceutical and offshore industries).
The correct selection of linear motion products is essential for design longevity. It is important to consider all of the above aspects before choosing the type of rolling element (ball, roller or plain) and then specifying the final product. Whilst cost will always have a major influence, product suitability is key.