Archive for the ‘Bearing Lubrication’ Category

Lubrication

Tuesday, March 17th, 2015

In any bearing the purpose of lubrication is to prevent contact between the rolling elements and the races or between the shaft and a plain bearing. The lubricant is necessary, being composed of the proper additives and of the proper viscosity. The correct lubricant to be used is determined by the speed and load carried by the bearings. Other than special cases, grease and oil are the most common lubricants. Your SKF bearings dealer can provide you with the data needed to determine the proper lubricant for your application.

For oils, viscosity is the prime determination in the selection of a lubricant. High or low temperature and speed determine the required viscosity. The lubricant forms a thin film between the races and the rolling elements. Insufficient viscosity will allow the two surfaces to make contact generating heat, wear and surface degradation.

Once a lubricant is selected it is just as important to use the proper amount. Excess lubricant will generate heat and can cause damage to the bearings. Bearing cages and rolling elements are not supposed to plow through grease or oil bath. In the case of electric motor bearings, excess lubricant will often escape into the motor housing causing a short in the electrical components. Some studies have shown that more bearings are damaged by excessive lubricant than those that lack a lubricant. SKF will also provide you with data on the amount of lubricant needed, based on your application.

Figure 1

Figure 1

Storage of lubricants is very important. If a storage drum is exposed to conditions where water can mix with the lubricant, degradation will occur. At standstill, free water in the lubricant will accumulate at the bottom of the bearing. The water concentration will be highest at a certain distance from the rolling contact (fig 1). The reason is that the free water is heavier than the oil and will sink until it comes to a suitable gap between the rolling element and the raceway. This can lead to deep-seated corrosion, called etching (fig 2). Etching is even more likely to occur in applications where there are aggressive chemicals and high temperatures, like the dryer section of a paper machine.
Figure 2

Figure 2

Etching usually leads to premature, extended spalling as the material is subjected to a structural change and the surfaces in the load zone are reduced to such an extent that overloading occurs. The best way to avoid corrosion is to keep the lubricant free from water and aggressive liquids by adequately sealing the application. Using a lubricant with good rust-inhibiting properties also helps.

 

 

Keeping Good Records

Tuesday, November 11th, 2014

One cause of bearing failure that often occurs and leaves few clues is the failure caused by greasing a sealed bearing. Over the years we have seen a number of cases where the vibration history of the bearing gives no clue as to what could cause the failure but when an autopsy of the bearing pieces is conducted, all we have is a jumbled mess of a twisted cage and loose balls.

The one common factor is that these bearings were sealed bearings; and the purpose of the title of this blog is, do you know which bearings on your machines have integral seals?

What often occurs in manufacturing plants is that the newest member of the maintenance group is often given the task of checking the grease and oil levels on the equipment. Minimum training, which usually consists of telling the person where the lubricants are stored and such broad instructions as “keep the oil level to this line”, and “just put two squirts of grease in each bearing” is all that is provided.

If the lube “specialist” is like most folks, then if two squirts of grease is good, then three or four will be better. WRONG.

If they are sealed bearings, then they do not need ANY additional grease. They are filled and sealed at the factory and any additional grease is both wasted and damaging. Damaging from the standpoint that grease guns can develop over 1,000 PSI and this will force the seal inward and interfere with the rolling elements. Shortly afterwards the bearing fails and the inspection shows there was plenty of grease, and everyone wonders why the bearing failed. The machine records must show whether sealed or open bearings are installed, and this information must be made available to the lube specialist.

The other reason that three or four squirts of lubricant is not good is that even for open bearings, that require periodic greasing, any open bearing is designed for the certain amount of lubricant. Any excess is wasted and will cause the bearing to overheat and be damaged. When the bearing is greased a measured amount should be applied on a scheduled basis.

Static Oil Bearing Lubrication

Tuesday, August 14th, 2012

We get fairly frequent calls from end users reporting failed bearings in wet sump, static oil lubricated applications shortly after startup. Inside, we find extremely dry, dead bearings, often with cages or rings melted into each other. The root cause may be simple: the machine was never lubricated prior to startup. In the USA, you’re generally not allowed to ship machines prefilled with oil these days. Many vertical motors, some gearboxes, fan asssemblies and so on are practically dry on delivery. It’s imperitave to check and fill these machines with the proper amount and type of oil before startup.

Insufficient Lubrication

Excessive Lubricant

 

If there is no obvious oil level guidance (marked sight gauges, dipsticks, etc.) on the outside of the machine, here are some guidelines for wet sump, static oil fill levels:

  1. Horizontal shaft ball and roller bearings: fill to the center of the lowest roller.
    1. In higher speed applications, the oil level may be reduced, but should always be at least 1mm (0.040″) above the bearing outer ring land.
  2. Vertical shaft,  single row ball bearings: fill halfway up the bearing width.
  3. Vertical shaft, double row ball and roller bearings: submerge the bearing in oil. (Static oil is generally not recommended in these machines.)
  4. Vertical shaft, spherical roller thrust bearings: fill to 60-80% of the housing washer height.

If the original shaft orientation is changed, or if application speeds or temperatures increase, it may be necessary to change to circulating oil, oil mist or other lubricant delivery methods. If multiple bearings are involved, as in some vertical motors with matched sets of angular contact bearings, it’s best to contact your local Applications Engineering Service for assistance.

Monitoring closely a newly started up machine or after a oil change through is recommended. Consider temperature measurement, vibration control and noise.

If you’re not going to put the machine into service right away, research and deploy suitable preservation strategies.

Point or Line Contact?

Monday, August 13th, 2012

Rolling elements in bearings make a very small area of contact with the raceways, sometimes smaller than 1 square mm (0.0015 sq. inches.) in ball bearings.  At the point of contact, there is a fundamental difference depending on the bearing type: “point” contact or “line” contact. Ball bearings generally have point contact, while roller bearings have line contact. The difference in operation is critical: in an electric motor with a ball bearing on one end and a roller bearing on the other, you’ll need to relubricate the roller bearing about twice as often as the ball bearing.

One advantage for roller bearings is that by spreading the load over a greater area, they can last longer, or they can be subjected to a greater load over time. This is why “sheave duty” motors often have a cylindrical roller bearing on the drive end. The belts in the pulley add a larger radial load to the roller bearing. The line contact spreads the load across a greater area, reducing the stresses on the bearing steel that could eventually lead to fatigue failure.

The terms “point” and “line” contact are somewhat inaccurate, since the contacts are actually small ellipses, and the area of contact varies depending on the load and other conditions. When you tear down and examine the raceways in damaged bearings, understanding the type of contact will help. The patterns created on the raceways and rolling elements change depending on the type of roller to raceway contact.

Replacing Grease

Wednesday, June 22nd, 2011

Last post I talked about grease compatability, and how it’s not a good idea to mix greases. But how do you get rid of old Grease? Basically, you wash it out.

Specific techniques, from the SKF Bearing Maintenance Handbook:

Renewal
Renewal is the process of stopping a machine, removing the existing grease inside the bearing
arrangement and replacing it with fresh grease. Renewing the grease fill is generally recommended after several replenishments or when the relubrication interval is longer than six months. When renewing the grease fill in a bearing arrangement with a split housing, SKF recommends the following:

  1. Clean the work area.
  2. Open the housing.
  3. Remove the used grease in the housing cavity completely, using a palette knife, and clean the housing cavity with a solvent.
  4. Clean the bearing with solvent and allow it to dry. Remaining traces of the solvent will evaporate.
  5. Fill the free space between the rolling elements and cage with grease from the accessible side, using a grease packer.
  6. Fill 30 to 50% of the housing with grease (typical quantity for normal applications).
  7. Put the housing cap back in position.
  8. Run-in the bearing.

When housings are not easily accessible but are provided with grease fittings and a grease escape hole, SKF recommends the following (Caution: CHECK COMPATABILITY):

  1. Make sure the grease escape hole is open.
  2. Clean the grease fitting.
  3. Introduce fresh grease steadily (not too fast) via the grease fitting, while the machine is operating.
  4. Capture the old grease expelled from the escape hole in a container.
  5. Continue to add fresh grease until fresh grease is expelled from the escape hole.

CAUTION: Adding too much grease or too quickly without the ability to purge will result in churning and high operating temperatures.

Grease more than a year old, especially operating at higher temperatures, is likely to require complete removal – purging may not displace it from the housing.

Grease Compatability

Thursday, June 9th, 2011

Greases are compatible? Do you believe it?

In attempting to keep up with various grease manufacturers’ opinions on grease compatability, I look to their websites, technical notes and published catalogs. Recently, I noted a major grease manufacturer had removed their grease compatability chart from their catalog. They substituted a comment that I’ll paraphrase as “we don’t recommend mixing greases.” I think this was a good move on the part of the manufacturer, and it’s an opinion I share. Don’t mix greases. It’s just better to clean out the housing, are replace the old grease with new. More on that later.

What is grease made of anyway? The first time I looked at grease, I thought to myself “it’s got to be half oil, and half thickener, just to hold everything together.”  Wrong! I found later that grease is more like custard or gelatin: it’s a chemical “mesh” holding the oil in place. Grease is typically about 90-95% oil, mixed with a thickener. The other component of grease is the additive package: rust and oxidation inhibitors, at a minimum, and almost certainly other additives for the particular grease in question.

Grease thickeners: Compatible? Not at all. The two most commonly used greases, Lithium complex and Polyurea, are generally incompatible.
Oils: Compatible?: Nope. Mineral oil and Polyglycol aren’t.
Additives: Compatible?: Don’t even think about it. Anti-wear additives may have severe chemical reactions with R&O (Rust and Oxidation) additives, for example.

But I can use the ASTM D6185 (1997) grease compatability test, and I’m covered, right? Wrong. This tests only covers dropping point, shear stability and storage issues. Check out the well-written caveats at the ASTM Site: http://www.astm.org/Standards/D6185.htm

Bottom Line: don’t mix greases. If you have to, be prepared for the consequences, which could include liquifaction, high bleed rates, grease softening or hardening and possibly even corrosion issues.

Question of the Month 2011-03

Wednesday, March 16th, 2011

This month it’s multiple choice:

What is the proper initial amount of grease for a Single Row, Deep Groove Ball Bearing, open (no seals or shields), running at moderate speed?

A. Fill the bearing to 100% with grease, housing to 40%

B. Fill the bearing and housing 100% with grease

C. Fill the bearing to 20% with grease, and the housing to 40%

D. Fill the bearing to 40% with grease and the housing to 100%

Answer : A: Fill the bearing to 100% with grease, housing to 40%

If you want to know why, Leave a comment!

Bearing Life and Lubrication

Tuesday, February 15th, 2011

Bearing life for most industrial applications is typically based on a 10% chance of failure. It’s quite conservative, since the average bearing life is 5 times as long. You can buy electric motors with 1, 3 or 5 year warranties. How does a bearing have enough grease to last 5 years?

  1. A five year warranty motor needs an expected service life of about 45,000 hours (5 years x 24 hrs per day x 365 days per year.) The one year warranty motor needs under 9,000 hours expected life. The bearings are larger, and there is more room for grease in the sealed bearing.
  2. The grease life for bearings is selected with a 1% chance of failure – so, if the motor is run properly, the bearing should fatigue fail before it runs out of grease.
  3. Rolling element bearings need a very thin oil film to operate – under 1 micron thick. (it takes about 25 microns to equal 0.001″)  The oil demand from the grease pack is so tiny, bearings operate reliably for years on their initial grease pack.
  4. Greases have improved significantly in the last 25 years: better oil, better grease soap base formulations, better additives.

What can you do to extend grease life?

Mainly, think about temperature.

Cool running strategies:

  1. Operate machines within their design envelope.
  2. Reduce parasitic loads on machines with precision alignment and balancing.
  3. Make sure there is fresh grease in the cavity surrounding sealed and shielded bearings. This will promote heat transfer to the housing and out of the machine. Yes, this means you’ll have to repack every 6-12 months. It’s called Best Practice.
  4. Don’t overpack the housing – grease needs 10-20% room to expand in most machines.
  5. Don’t try to relube sealed and shielded bearings. Too many things can go wrong.

Question of the Month: How much oil?

Thursday, January 20th, 2011

What is the proper static oil level for an open single row deep groove ball bearing, mounted on a horizontal shaft?

Fill the housing with oil:

  • a. to cover the bearing with oil
  • b. to the centerline of the shaft
  • c. to the bottom of the shaft
  • d.  The centerline of the lowest rolling element

The correct answer is d. A higher oil level, especially in higher speed applications, often creates more heat. This can reduce the oil viscosity even further,  leading to metal-to-metal contact in the bearing, and a reduction in bearing service life.

If higher speeds are required, investigate circulating oil, oil mist or oil spot delivery methods for the lubricant.

Leave some space for your grease

Monday, November 29th, 2010

Typically, open bearings using grease lubrication should be filled 100%, and the free space in the housing should be filled 50%. This leaves room for the bearing to eject any excess grease, while at the same time leaving room for the grease to expand. Greases can expand up to 10% of their volume with a temperature rise of 50°C, which is not unusual in many bearing applications. Since grease is usually about 90% oil, this means we have good contact with the bearing, which promotes heat transfer.

For slow-moving applications with little temperature rise expected, filling the bearing and housing 100% may be acceptable – the grease can act as a barrier to contaminants.

Next post we’ll dive into grease selection, considering speed, load and temperature.