Alloy bushes perform better than steel
April 28, 2006 – 10:53 amTo solve a recurring problem with steel sleeve bushes on continuous mining machines, Joy Manufacturing has switched to a novel alloy. Since then, there been no failures, says the company.
Underground mining is one of the most demanding applications for bearings. No matter how corrosive the environment in a salt mine, or how wet and aggressive in a coal mine, and regardless of whether or not a maintenance schedule is strictly adhered to, bearings need to fulfil their function by ensuring low friction and minimal wear. To solve a recurring problem with steel sleeve bushes on continuous mining machines, Joy Manufacturing has switched to Brush Wellman’s unique ToughMet alloy.
Since then, there have been no reported seizures, bearing failures, or premature wear.
Continuous mining machines are substantial pieces of equipment, used for mining salt, coal and other minerals.
Traditionally the dynamic pins have been mounted in bushings manufactured from steel to provide the required strength, with complex lubrication systems and, typically, figure-of-eight channels machined into the bushes to distribute the lubricant.
So long as the lubrication system is operating correctly, the design usually gives good results, but underground mining is an environment where regular maintenance is difficult and, therefore, not always carried out at the appropriate intervals.
As a result of inadequate lubrication, galling often occurs between the steel bush and the steel pin, and the two parts occasionally become inseparable due to a build-up of corrosion products.
Alternative materials that Joy has previously investigated for the bush include bronzes - which suffer if contaminated - and copper beryllium.
However, neither material type has the strength of the steel bush, which means that a substantial redesign of the bearing would be required.
The major benefit offered by ToughMet, a spinodal alloy of copper, nickel and tin, is that it has the strength of steel and the low-friction and lubricity more usually associated with leaded bronzes.
This means that a compact bearing arrangement can be designed, and that the bearing will continue to function even if starved of lubricant.
For the dynamic pin bearing application, Joy Manufacturing has specified Brush Wellman’s ToughMet 3 CX 105, a cast alloy with 15% nickel and 8% tin, giving a yield strength of 105000 psi (724 MPa); the mating component is a carburised steel pin.
One of the problems with heavy mining equipment is that the bearings are always loaded, so it is virtually impossible to get lubricant to the point where it is actually needed.
This is especially so for oscillating joints, where heat can quickly build up and spoil the effect of any heat treatment that may have been used.
For ToughMet, however, the inherent lubricity means that lubricant starvation is not disastrous and that high temperatures do not occur.
Because the fit between the pin and bushing is different for ToughMet than for steel, Joy has developed a standardised pin-bush fit that can be used in all cases where a ToughMet bearing needs to be designed.
So far the company has concentrated on redesigning bearings that have traditionally proved to be problematic, but the alloy is now being incorporated in new applications where it is felt to be desirable to avoid using steel bushings.
Joy receives the ToughMet in the form of standard-sized cast tube, so the amount of machining is minimised and only a small proportion of the valuable material is wasted as swarf.
Looking to the future, Joy is aiming to design all of its ToughMet bushes so that they can be machined from just two customised sizes of tube, thereby optimising the machining time (and swarf generation) and making it very cost-effective for Brush Wellman to produce casting runs to order.
Typical Joy bushings range in size from 1.5-inch bore with 0.25-inch wall thickness to 6-inch bore with 0.625-inch wall thickness.
Although ToughMet is more costly per tonne than steel, there is scope for making savings elsewhere.
For example, steel bushings need complex machining operations to produce lubricant grooves, and these are not required for ToughMet bushings.
In the long run, it may also be possible for ToughMet to be used instead of steel for large numbers of bushings, thereby reducing the size and complexity of the bearing lubrication system or, ultimately, to eliminate it altogether.
The concept of a continuous mining machine that has lube-for-life ToughMet bearings is also attractive to mine operators who are aware of the high cost of underground maintenance in terms of both the maintenance procedures and the downtime.
Chris Stewart, Senior Engineer for Joy Manufacturing’s continuous miner development group, comments: ‘There have been no reported failures of our ToughMet bushings and the test rig that we developed has shown that the material is far superior to any of the alternatives currently available.
The combination of high strength and low friction is very unusual and is just what we need for replacing problematic steel bushings.
Moreover, the alloy’s other properties - such as toughness, corrosion resistance and the ability to withstand particulate contamination - are also highly advantageous.’ Stewart goes on to praise the support that his company has received from Brush Wellman: ‘We were already using Brush Wellman’s copper beryllium alloys when they introduced us to ToughMet.
The potential benefits for replacing some of the steel bushings were immediately apparent to us so we instigated a development programme; the support we have received from Brush Wellman right throughout that project has been excellent.
In addition, we have always been able to obtain the alloy in the correct grade, with no need to purchase large minimum order quantities.
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