Disadvantages of re-greasing of bearings during conventional Preventive Maintenance Program

Let us understand first of all why the bearings need re-greasing? To know this, we should know the purpose of grease in grease-lubricated bearings.

The purpose of grease is,

• To Lubricate each part of the bearing to reduce friction,
• To reduce Wear & fatigue, heat dissipation,
• To Cover each contact surface between bearing sleeves & rolling elements & other parts like a cage, with proper Oil Film
• To prevent Corrosion in the parts of bearings
• To prevent contamination due to dust to reach contact surfaces of bearing

To solve these purposes it is important that the chemical composition, density & properties of grease should remain as it is throughout its usage. 

But in reality, the properties of the grease in the bearings get deteriorated throughout the usage. The grease is undergoing a churning effect; it is continuously subjected to heat produced in the bearing, and ingress of water/air inside the bearing housing, deteriorates the properties of grease.

Now it is easy to know that, with the changed properties of grease, the purpose of grease will never be fulfilled and the costly bearings would fail. Consequently, the machine on which these bearings are installed will go under breakdown & which will lead to a stoppage of the plant, and manufacturing process and hence, the downtime will increase.

Therefore, the necessity for the replacement of grease arises whenever the property of grease deteriorates. 

We all know that Re-Greasing of bearings of equipment in a company is done periodically, i.e. during Scheduled Preventive Maintenance Program or Time Based Preventive Maintenance Program.

Let us recollect the steps of planning/ Scheduling Preventive Maintenance.

1. The first condition of the Preventive Maintenance Program is the plant has to be under SHUT DOWN during this PM period. Hence, the dates & period of Shut Down of the plant have to be decided first. Normally before monsoon season, this shut down is being taken.
2. As one cannot keep a plant closed for a longer period this shut down cannot be prolonged for more than 2 – 3 days.
3. The bearings under PM are listed out. Normally all the machines/equipment is taken under the Preventive Maintenance Program.

Now let us write down a condition here, a normal size plant has around 1500 – 2000 equipment/ machines in the plant. Hence, re-greasing of 3000-4000 bearings is to be done within the stipulated time period of 3 days with limited manpower & with limited apparatus & limited sources. 

What happens when we do re-greasing of several bearings in the scheduled PM program within a given time frame? Disadvantages,

1. Cost of Material (Grease) – Re-greasing is done in all the bearings, the company is spending a good amount of money on the purchase of good quality Grease. In reality, the bearings re-greasing can be managed with a much lesser grease amount & the inventory of the grease/ spending on grease can be reduced. How?

All the bearings necessarily do not need re-greasing. The grease property/quality/quantity in many bearings remains good. It is not necessary to do re-greasing in these bearings. 50 – 60% of bearings among all the bearings under the PM program do not need replacement of grease or re-greasing.

1. Quality of Workmanship – Although we have a team of expert technicians, they have a target of completing the job of re-greasing of 2000 – 3000 bearings within a closed timeframe of 2-3 days.

 Here we need to understand what an ideal re-greasing process should be. Let us learn it pointwise,

1. The new grease that is injected into a bearing housing with the help of a grease gun shouldn’t have any contamination of dust, carbon, water, etc.
2. The old grease in the bearing housing has to be drained out/removed completely.
3. The correct quantity of grease should be filled in bearing housing. No Over-Greasing. No Under-Greasing.
4. Correct Quality – Grade – Specification of grease should be used.
5. Mixing of two different grades of greases shouldn’t be done.

But what actually happens when the technicians do this job in hurry?

1. They need to fill the grease gun frequently after emptying it during a day’s work. Whenever they fill the grease gun they use their hands. They collect the grease from the Grease drum by hand and then fill the grease gun. Do you think, that every time when they fill the grease in the grease gun, they are washing their hands? Certainly not. They don’t have time to keep washing their hands during many intervals of filling grease in a grease gun, as they have to re-grease thousands of bearings within a narrow time interval. Their hands contain carbon dust, and mud and they use the same hands to fill the grease. Hence, the grease gets contaminated in the first place, even before filling in the grease gun.
2. They carry the grease drum along with them and mostly the drum is kept open throughout the day. What happens with the grease in those drums? The dust & carbon particles in the air keep on depositing on the grease in the drum and contaminate it.
3. The technicians don’t know the exact quantity to be injected into the bearing cartridge. Hence, over-greasing or under-greasing is likely possible in many cases/ bearings.
4. Many times technicians aren’t much bothered about mixing different grease. They may carry different grade grease in the same grease drum they were using since the beginning of the PM program. That results in the mixing of the grease which changes the properties of grease.
5. The technicians may use a different grade grease instead of the one they should for re-greasing which results in unsuitable greasing of some bearings. Although this happens rarely as the technicians are experienced & know about their machines very well, even the rarely & mistakenly re-greasing the bearings may result in damaging the bearings.
6. Design of Bearing Assembly – The grease drain plug in all grease-lubricated bearing cartridges is a must so that while greasing the excess grease can be drained out from the bearing cartridge and the bearing can be saved from over-greasing.

 But in reality, many equipment manufacturers do not consider issues with maintenance while designing their equipment/machines and they leave their bearing cartridges without the provision of a Grease Drain plug.

If one injects grease through a grease gun in such bearings, the old grease remains inside the cartridge and we add new grease to it. It turns into two fault conditions, Over-Greasing & Mixing of Grease, which is always harmful to the bearings.

1. Downtime due to Shutdown of plant – The major disadvantage of Scheduled or Time Based Preventive Maintenance (TBPM) is DOWNTIME. The plant needs a shut down for a few days and that interrupts the production process and affects the uptime management. The major & time-consuming work under TBPM is always Re-greasing the bearings.

So, Can we restrict the disadvantages re-greasing of bearings & can we reduce/eliminate the shutdown period? The answer is YES!!!

If we adopt Condition Based Preventive Maintenance (CBPM) that too with a controlled Acoustic Greasing Method in place of Time Based Preventive Maintenance (TBPM), we can eliminate almost all the disadvantages listed above.

You can read & learn in detail about CBPM & Acoustic Greasing elsewhere on this website. Let us learn about acoustic greasing in very brief here. The acoustic greasing is done online and shutdown of the machine/equipment is not required at all. As this is Ultrasound Guided greasing the grease is added in only those bearings where ever it is required and to fill exact quantity is very much possible, which eliminates chances of bearing getting over-greased. The grease contamination can be detected easily during the acoustic greasing process and hence, the same can be avoided.

This is all about the disadvantages of Re-greasing during conventional PM program. If you have any questions or queries, please write in the comment box and we will reply to you.