Sooty09
Registered user
Don't pack the bearings as in cram as much in as you can, leave space for the grease to move about. Too much and it will cavitate letting the balls revolve in a gap in the grease unlubricated.The manufacturers don't put so little in to save on cost, its the optimum amount for general use.
I have read a lot of comments about replacement bearings lasting only a few thousand miles, I find it hard to believe any bearing manufacturer is that bad. I suspect the new bearings are being hammered in using the inner race. Don't ever do this, chill the bearings in the freezer, warm the hub with hot water and press them in using a large socket on the outer race only.
The following is an extract from a bearing design paper (in this case a centifugal pump)
It turns out that bearings fail for two main reasons:
Contamination of the bearing lubricant by water or moisture.
High heat often caused by too much lubrication.
As little as 0.002% water in the bearing oil will reduce bearing life 48%. The water enters from packing leakage, wash down hoses and aspiration caused by the temperature cooling down in the bearing casing after shutdown, and moisture laden air entering the bearing case. A 6% water content in the oil will reduce bearing life by as much as 83%. The water or moisture contamination comes from three main sources:
Packing leakage.
Water hoses used to wash down the base plate area because of packing leakage.
Aspiration or moisture in the air entering the bearing case especially when the pump is stopped.
In paper 13-9 we talked about the seals you can use to keep this moisture out of your bearing case. In this paper we will investigate the second reason bearings fail.
Excessive heat!
A couple of paragraphs above I said that over lubrication would cause high heat. What is the problem with over lubrication? If a little lubrication were good, wouldn't a lot be better? Not really! Think about it this way. Picture yourself on a hot day walking along the beach. You go into the water up to your ankles, and as you walk along rapidly you feel cool and refreshed. Now walk rapidly in water up to your waist and you see the problem. It takes a lot of energy to get through the same temperature water and this would make you hot and fatigued instead of cool and refreshed
It's the same thing with lubrication. Too high a lubrication level and the bearing will consume energy as it plows through the lubricant. This energy will show up as heat added to the lubricant causing it to first lose its viscosity and then the lubricant will begin to form varnish and coke as it gets hotter. Varnish and coke are another name for solids.
The problem with grease and oil lubricants is their low specific heat and their poor conductivity. Some of the synthetics are better, but they have a temperature limit that is still too low for many pumping applications. It is for this same reason that we do not recommend putting any type oil between dual seals if we can avoid it.
The SKF bearing company claims that uncontaminated grease and oil has a useful life of thirty years at 30°C (86°F) They further state that the life of grease and oil is cut in half for each 10°C (18°F) rise in temperature. That means that at 100°C (212°F) oil and grease have a useful life of only 90 days. Here are your lubrication options:
Grease packed
Grease is hard to change because the usual method is to pump grease into a grease fitting and let the new grease push out the old grease. This method guarantees the bearing will be over lubricated.
The only proper way to grease a bearing is to hand pack it full, but not the cavity where it is located. As the bearing heats up some of the grease will leak into the cavity reducing the amount of lubrication in the bearing.
I have read a lot of comments about replacement bearings lasting only a few thousand miles, I find it hard to believe any bearing manufacturer is that bad. I suspect the new bearings are being hammered in using the inner race. Don't ever do this, chill the bearings in the freezer, warm the hub with hot water and press them in using a large socket on the outer race only.
The following is an extract from a bearing design paper (in this case a centifugal pump)
It turns out that bearings fail for two main reasons:
Contamination of the bearing lubricant by water or moisture.
High heat often caused by too much lubrication.
As little as 0.002% water in the bearing oil will reduce bearing life 48%. The water enters from packing leakage, wash down hoses and aspiration caused by the temperature cooling down in the bearing casing after shutdown, and moisture laden air entering the bearing case. A 6% water content in the oil will reduce bearing life by as much as 83%. The water or moisture contamination comes from three main sources:
Packing leakage.
Water hoses used to wash down the base plate area because of packing leakage.
Aspiration or moisture in the air entering the bearing case especially when the pump is stopped.
In paper 13-9 we talked about the seals you can use to keep this moisture out of your bearing case. In this paper we will investigate the second reason bearings fail.
Excessive heat!
A couple of paragraphs above I said that over lubrication would cause high heat. What is the problem with over lubrication? If a little lubrication were good, wouldn't a lot be better? Not really! Think about it this way. Picture yourself on a hot day walking along the beach. You go into the water up to your ankles, and as you walk along rapidly you feel cool and refreshed. Now walk rapidly in water up to your waist and you see the problem. It takes a lot of energy to get through the same temperature water and this would make you hot and fatigued instead of cool and refreshed
It's the same thing with lubrication. Too high a lubrication level and the bearing will consume energy as it plows through the lubricant. This energy will show up as heat added to the lubricant causing it to first lose its viscosity and then the lubricant will begin to form varnish and coke as it gets hotter. Varnish and coke are another name for solids.
The problem with grease and oil lubricants is their low specific heat and their poor conductivity. Some of the synthetics are better, but they have a temperature limit that is still too low for many pumping applications. It is for this same reason that we do not recommend putting any type oil between dual seals if we can avoid it.
The SKF bearing company claims that uncontaminated grease and oil has a useful life of thirty years at 30°C (86°F) They further state that the life of grease and oil is cut in half for each 10°C (18°F) rise in temperature. That means that at 100°C (212°F) oil and grease have a useful life of only 90 days. Here are your lubrication options:
Grease packed
Grease is hard to change because the usual method is to pump grease into a grease fitting and let the new grease push out the old grease. This method guarantees the bearing will be over lubricated.
The only proper way to grease a bearing is to hand pack it full, but not the cavity where it is located. As the bearing heats up some of the grease will leak into the cavity reducing the amount of lubrication in the bearing.
!
