A huge amount of technical bearing data and procedure is offered by most bearing manufacturers. If you are like me, you simply do not have the time, patience, or interest, in wading through all of this information, with cost effectiveness being unsupported by the frequent need to outsource bearing maintenance, but yet do have an interest in running cost reduction for your tree stump grinder. Replacement bearings cost over £100 each, and represent a large portion of overall machine maintenance costs. This guide is meant for those who wish to reduce cost by increasing bearing service life. Bearing failure or premature bearing wear on stump grinders is mainly due to inadequate maintenance of clean lubricant. I use a pneumatic grease dispenser driven by an air compressor at each days end of stump grinding, with a grease top up, out in the field, using a conventional hand-held grease pump. I use a great deal of grease but it is far cheaper than replacement bearings. When I re-grease bearings, I thoroughly purge out the contaminated grease, until clean grease exudes from the bearing. A high melting point EP-2 grease is essential. I prefer a molybdenum disulphide (moly) enriched EP-2 complex grease because it is a heavy-duty grease. So, to sum up this paragraph, there are three principles of maximizing bearing life insofar as lubricants are concerned:
- select the correct grease
- apply the correct grease properly e.g in the correct quantity
- re-grease frequently
A second common reason for premature bearing wear on stump grinders is inadequate or incorrect bearing adjustment ( for those machines with an adjustable bearing wear takeup) Now, I know how awkward accessing bearings for adjustment, can be on stump grinders. I made up some specialised tools out of allen keys, owing to their tough, high quality steel content, as a work around to the awkward access problem I have on my grinder. I simply ground away excess material with a bench grinder to shape an allen key into a right-angled punch. If you are comfortable doing this, please be sure to frequently cool the allen key in water. Do not let it turn blue in colour. I come from an engineering background, and accept that many who may read this, do not. Many may feel this to be somewhat out of their depth. This is why I’m making this effort to avoid technicalities as much as possible with emphasis on minimum salient points. I’m not conversant with bearing access configurations on all brands of stump grinders, but I can’t over emphasis how, with a bit of initiative and determination, the importance to overcome a bearing access problem is. It will save you money.
The detection and analysis of premature bearing failure is by its nature a technical subject involving specialised equipment e.g. heat sensors, vibration sensors, infrared sensors, oil and ultrasound analysis equipment. All of this equipment generates a great deal of data, is very costly, and totally impractical out in the field. A non technical tree stump grinder who is dependent upon the continual operation of his stump grinding machine will easily be put off by this situation. The easiest, most practical and non technical way to roughly monitor bearing performance is to rest your hand on the pillow block bearing after running it for a reasonable length of time, say at least one hour. If you must remove your hand because the bearing is too hot, this indicates a worn bearing. Adjustment to take up excessive play within the bearing be made in accordance with manufacturers instructions. If the sound of a bearing “knock or rumble” is also heard, this usually means it is excessively worn and needs replacement. For those who my be interested, there are four stages of bearing failure which I will touch on, as it does get involved and is not in keeping with this scope and its objective.:
- Stage 1. Vibration analysis detects the earliest sign of bearing failure. Signal data appears in the ultrasonic frequency range band around 250 to 350 Kilo Hz.. At this point there is about 10-20 percent remaining bearing life.
- Stage 2. Bearing failure begins to “ring” at its natural frequency (500 – 2000 Hz) At this point there is 5 to 10% bearing life left.
- Stage 3. bearing failure harmonics are apparent with defects in the inner and outer races. Temperature increase is now apparent with 1 to 5% bearing life left
- Stage 4. High vibration is clear with random ultrasonic noise greatly increased, temperature increases quickly. One hour to one percent remaining life.
It is unfortunate for all stump grinder operators, that by the time a bearing knocks, rumbles, or noticeably heats up, it is shortly on its way to failure. There is no practical way for the average owner / operator to accurately judge the length of serviceable time remaining in a stump grinders bearing while out in the field.
From this viewpoint, I am making an attempt to try to collect some useable data out in the field, which may prove to be a practical and inexpensive way to monitor bearing serviceability. I will be installing thermocouples (heat sensors) to the bearings. If a running average temperature gradient can be established, it may prove helpful to those stump grinders who are concerned with maximizing their machines bearing life, by simply adjusting up their bearings, when its temperature rise indicates time to do so. This will require some effort on the part of those interested. They too will need to fit thermocouples to their bearings to monitor temperature as the only cost-effective, and practical way to know when to adjust up their bearings.
This effort will be totally negated if a strict and sufficient re-greasing program is not maintained. What would be the point of regular, timely, bearing adjustments, if the bearings regularly ran in a bed of contaminated grease. I think the reader will agree, is a total waste of time and effort.