Sunday, 10 May 2015

8 WAYS TO PREVENT BEARING FAILURES

8 WAYS TO PREVENT BEARING FAILURES

Today, I want to share few interesting points about the bearings.

We all are well aware that there should be a micro thin film of lubricant in between rolling element and bearing raceway, in order to reduce the friction between rolling element and raceway of bearing as well as to avoid corrosion action over any element of bearing.

So what we will secure here and why should we read this post??

We will try to find out here the mistakes in bearing lubrication, we must note it that mistakes that we will discuss here might be avoided if we know the basic of lubrication and with little budget. It is good for our machine health if we understand the mistakes and the way by which we can avoid these mistakes.

1.    Excess lubrication

Lubrication is very important for the desired service life of bearing but we must understand that quantity of grease or oil must not exceeds the limit as excessive quantity of grease or oil in bearing or in bearing housing will yield rise in temperature. We may fill the bearing with grease completely but fill grease in bearing housing to only 1/3rd portion. Let us consider the case of grease lubricated bearing, when machine will start filled grease in bearing will be splashed in housing and hence there must be suitable space for accommodating the splashed grease from bearing during starting of bearing.

2.    Insufficient lubrication

Insufficient lubrication of bearing is also not acceptable for the health of our machine bearing. Less quantity of lubricant (grease or oil as per recommendation) will also yield the rise in temperature as due to insufficient lubrication there will be metal to metal contact between rolling element and bearing raceway.

I have observed one case of bearing failure due to insufficient lubrication, two bearings were fixed at both end of a rotor of a shredding unit and bearings were oil lubricated, oil leakages were observed from bearing housing and 200 liters of hydraulic oil 46 were wasted as leak oil from bearing housing in 8 running hours of machine.

Maintenance crew team decreased the flow of hydraulic oil to reduce the leak oil but this decision was very danger for the health of bearing and one bearing out of two bearing was failed due to continuous operation of machine under insufficient lubrication.


Root cause analysis team have secured the root cause that drain line of bearing housing was chocked and hence oil was coming through slightly V grooved oil seals of bearings instead of draining via drain line. Finally drain line was cleaned and oil seals were replaced with new bearing and clear instructions were given to team to not reduce the flow of hydraulic oil. 

Before replacing oil seal and cleaning drain line, you can’t imagine how much quantity of hydraulic oil was wasted as leak oil from the system.

3.    Application of lubricant of correct viscosity

Lubricant selection criteria are based over the viscosity of oil or grease. As maintenance crew member, I am also using same grease as lubricant for all bearings in our organization. But if we follow the SOP of lubrication, there is different viscosity grade recommended for ball bearing, spherical roller bearing and thrust bearing.

Ball bearing: Lubricant grease viscosity 13 cst minimum
Spherical roller bearing: Lubricant grease viscosity 20.6 cst minimum
Thrust bearing: Lubricant grease viscosity 32 cst minimum

We normally use single lubricant for multiple bearing or for all bearing but we must understand that viscosity of lubricant vary inversely with temperature that means decreases with rise in temperature and hence we should not use lower grade viscosity grade lubricant in spherical roller bearing and thrust bearing.

4.    Contamination in lubricant

Contamination in lubricant (oil or grease) is very dangerous for service life of bearing similarly as acid for our face. Dust particle and other foreign particle enter in bearing raceway due to mishandling of bearing from opening of its cover to fixing with respective shafts. Dust particle also enters in bearing housing via improper fitted bearing cover or by damaged lubricant seals and dust particle may enter via grease point made in bearing housing if grease nipple is not available.


Dust particle deteriorate the rolling element, raceway of bearing as well as it also help to increase the bearing clearance. Bearing temperature might be high due to such dust particle; hence we must be careful during handling, installation and lubrication of the bearings.

5.    Oil seal

 
As we have studied in above point that dust particles might enter in to bearing housing and in between bearing rolling element and bearing raceways. Hence we must use oil seals of good quality such as, rubber metallic seal, viton seal and garter spring seals.

6.    Failure to inspect the bearing Clearance

We as disciplined maintenance crew team member usually follow the lubrication frequency as per recommended schedule. According to SKF we must inspect the bearing after opening its bearing housing cover, cleaning, recording of bearing radial internal clearance and filling fresh recommended grease once in year at least.

Allowable radial internal clearance limit of bearing = 0.006 to 0.02 mm

7.    Use the Correct lubricant

We know oil and grease both are using as lubricant in industries for various machineries; we must have to use grease and oil as per manufacturer recommendation only. If manufacturer recommendation is given to use oil, then we do not have to use grease there.

8.    Using mixed inappropriate lubricant

We must take care during re-lubrication of bearing that maintenance crew team use similar lubricant as earlier used with that bearing. Mixing of inappropriate grease, such as sodium based grease and lithium based grease, reduces the lubrication quality. 

Similarly for oil lubricant also, we should never mix mineral oil with synthetic oil in any case.

Awareness of lubrication of bearing

We normally attend training and seminars for various technical agendas such as “how to install bearing correctly”, we must have to trained our maintenance crew in lubrication also. Lubrication quality and result of lubrication will only be positive if technicians understand and feel the importance of lubrication.

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