KT0408: Maintenance requirements and methods for bushes and bearings

When a rolling bearing is damaged during the machine’s operation, the entire machine or equipment may seize or malfunction. Since bearings that fail prematurely or unexpectedly cause trouble, it is essential to be able to identify and predict failure, if possible, so that preventative measures can be adopted. Generally, bearing inspection or housing inspection can identify the cause of the problem. Often, the cause is attributable to poor lubrication, improper handling, selecting the wrong bearing, or not having studied the shaft and housing sufficiently. Usually, the cause can be determined by considering the operation of the bearing before the failure, investigating the lubrication conditions and the mounting condition, and carefully observing the damaged bearing itself.

1.         Bearing handling

1.1       Precautions for handling

Since rolling bearings are high-precision machine parts, they must be handled carefully. Even if high-quality bearings are used, their expected life and performance cannot be attained if they are misused. The main precautions to be observed are as follows:

• Keep the bearings and surrounding area clean
• Dust and dirt, even if invisible to the naked eye, have harmful effects on bearings. It is necessary to prevent the entry of dust and dirt by keeping the bearings and their environment as clean as possible.
• Careful handling
• Heavy shocks during handling may scratch or cause other damage to the bearing, possibly resulting in bearing failure. Substantial impacts may cause brinelling, breaking, or cracking.
• When preparing bearings for storage, wash and dry them, coat them with oil or another preservative, and wrap them in protective paper.
• Use proper tools
• Always use the proper tool when handling bearings and avoid general-purpose tools.
• Prevent corrosion
• Since sweat on the hands and various other contaminants may cause corrosion, keep your hands clean when handling bearings. Wear gloves if possible.

Sometimes, bearings are damaged and fail quickly and unexpectedly. Such premature failure is different from fatigue failure, which is due to flaking. Bearing life can be separated and categorised into two types: premature failure and normal rolling contact fatigue.

2.         Bearing mounting

Recommended preparation prior to mounting:

a.         Preparation of bearings

Wait until just before mounting before removing the bearings from their packaging to prevent contamination and rust. Since the anti-corrosion oil covering bearings are a highly capable lubricant, the oil should not be cleaned off if the bearings are pre-lubricated or when the bearings are used for normal operation. However, if the bearings are used in measuring instruments or at high rotation speed, the anti-corrosion oil should be removed using a clean detergent oil. After the removal of the anti-corrosion oil, bearings should not be left for a long time because they rust easily.

b.         Inspection of shafts and housings

• Clean up the shaft and housing to check whether it has flaws or burrs as a result of machining.
• Be very careful to completely remove lapping agents (SiC, Al2O3, etc.), casting sands, and chips from inside the housing.
• Next, check that the dimensions, forms, and finish conditions of the shaft and the housing are accurate to those specified on the drawing.
• The shaft diameter and housing bore diameter should be measured at several points, as shown

Measuring points on shaft diameter

Measuring points on housing bore diameter

Furthermore, the fillet radius of the shaft and housing and the squareness of the shoulders should be checked.

When using shafts and housings that have passed inspection, it is advisable to apply machine oil to each fitting surface just before mounting.

Mounting procedures depend on the type and fitting conditions of bearings. For general bearings in which the shaft rotates, an interference fit is applied to the inner rings, while a clearance fit is applied to the outer rings. For bearings in which the outer rings rotate, an interference fit is applied to the outer rings. Interference fitting is roughly classified as shown here. The detailed mounting processes are described in Tables 15-1 to 15-3.

“Table 15-1 Press fit of bearings with cylindrical bores”

“Table 15-2 Shrink fit of cylindrical bore bearings.”

“Table 15-3 Mounting bearings with tapered bores”

“Table 15-4 Mounting tapered bore spherical roller bearings.”

The force is necessary to press fit or remove bearing inner rings differs depending on the shafts’ finish and the interference the bearings allow. The standard values can be obtained by using the following equations.

Value of resistance coefficient ƒk

2.1       Press fit of bearings with cylindrical bores

2.2       Shrink fit of cylindrical bore bearings

Heating temperature and expansion of inner rings:

• Thick solid lines show the maximum interference value between bearings (class 0) and shafts (r6, p6, n6, m5, k5, j5) at average temperature.
• Therefore, the heating temperature should be selected to gain a more significant “expansion of the bore diameter” than the maximum interference values.

(When fitting class 0 bearings having a 90mm bore diameter to m5 shafts, this figure shows that heating temperature should be 40℃ higher than room temperature to produce expansion larger than the maximum interference value of 48μm. However, taking cooling during mounting into consideration, the temperature should be set 20 to 30℃ higher than the temperature initially required.)

2.3       Mounting bearings with tapered bores

2.4       Mounting tapered bore spherical roller bearings

Remark – The values for reduction of radial internal clearance listed above are values obtained when mounting bearings with CN clearance on solid shafts. In mounting bearings with C3 clearance, the maximum value listed above should be taken as the standard.

It is advisable to study the bearing mounting thoroughly since the quality of the bearing mounting influences the bearing’s running accuracy, life, and performance. It is recommended that the mounting method include the following steps:

• Clean the bearing and surrounding parts.
• Check the dimensions and finish conditions of related parts.
• Follow the mounting procedure.
• Check if the bearing is mounted correctly.
• Supply the correct kind and quantity of lubricant.

Since most bearings rotate with the shaft, the bearing mounting method is generally an interference (tight) fit for the inner ring and shaft while giving a clearance (loose) fit for the outer ring and housing.

3.         Bearing dismounting

After dismounting bearings, handling of the bearings and the various methods available for this should be considered. If the bearing is to be disposed of, any simple method, such as torch cutting, can be employed. If the bearing is to be reused or checked for the causes of its failure, the same amount of care as in mounting should be taken in dismounting so as not to damage the bearing and other parts. Since bearings with interference fits are easily damaged during dismounting, measures to prevent damage during dismounting must be incorporated into the design. It is recommended that dismounting devices be designed and manufactured, if necessary. It is helpful in discovering the causes of failures when the conditions of bearings, including mounting direction and location, are recorded prior to dismounting.

3.1       Dismounting of cylindrical bore bearings

3.2       Dismounting tapered bore bearings

3.3       Dismounting of outer rings

4.         Check the operation

After mounting the bearing, it is essential to carry out an operating test to confirm that it is mounted correctly. Table 1 indicates operating test methods. If irregularities are detected, immediately suspend the test and consult Table 2, which lists appropriate countermeasures to specific bearing problems.

Table 1 Methods to check operation

Table 2 Causes and countermeasures for operating irregularities

5.         Bearing maintenance

It is necessary to periodically inspect and maintain the bearing and its operating conditions in order to maximise the bearing life. In general, the following method is adopted:

5.1       Inspection under operating conditions

To determine the bearing replacement periods and replenishment intervals for lubricant, investigate the lubricant

properties and consider factors such as operating temperature, vibration, and bearing noise.

Inspection of the bearing

Be sure to investigate the bearing thoroughly during periodic machine inspection and part replacement.

• Check the raceway condition.
• Determine if damage exists.
• Confirm if the bearing can be reused or should be replaced.

15.2      Bearing performance factors

a.         Bearing noise

During operation, sound detection instruments (stethoscope, NSK Bearing Monitor, etc.) can be used to investigate the volume and characteristics of the bearing rotation noise. Bearing damage, such as small flaking, can be distinguished by its unusual yet characteristic noise.

b.         Bearing vibration

Bearing irregularities can be analysed by measuring the vibrations of an operating machine. A frequency spectrum analyser is used to measure the magnitude of vibration and the distribution of the frequencies. Test results enable the determination of the likely cause of the bearing irregularity. The measured data varies depending on the bearing’s operating conditions and the location of the vibration pickup. Therefore, the method requires determining evaluation standards for each measured machine. It is helpful to detect irregularities from the bearing vibration pattern during operation.

c.         Bearing temperature

Generally, the bearing temperature can be estimated from the temperature of the housing outside surface, but a

preferable way is to obtain direct measurements from the bearing outer ring by a probe going through an oil hole.

Usually, the bearing temperature gradually increases after the start of operation until reaching a steady state condition about 1 or 2 hours later. The bearing steady state temperature depends on load, rotational speed, and the machine’s heat transfer properties. Insufficient lubrication or improper mounting might cause the bearing temperature to rise rapidly. In such a case, suspend the machine’s operation and adopt an appropriate countermeasure.

16.       Effects of lubrication

The two primary purposes of lubrication are to minimise friction and reduce wear inside bearings that might otherwise fail prematurely. Lubrication provides the following advantages:

• To lubricate each part of the bearing, and to reduce friction and wear.
• To carry away heat generated inside the bearing due to friction and other causes.
• To cover the rolling contact surface with the proper oil film in order to prolong bearing fatigue life.
• To prevent corrosion and contamination by dirt.

17.       Maintenance and inspection of bearings

Periodic and thorough maintenance and inspection are indispensable to achieving full performance from bearings and lengthening their useful life. Besides, preventing accidents and downtime by early detection of failures through maintenance and inspection dramatically contributes to enhancing productivity and profitability.

17.1      Cleaning

Before dismounting a bearing for inspection, record its physical condition, including taking photographs. Cleaning should be done after checking the amount of remaining lubricant and collecting lubricant as a sample for examination.

• A dirty bearing should be cleaned using two cleaning processes, such as rough cleaning and finish cleaning.
• It is recommended that a net be set on the bottom of cleaning containers.
• In rough cleaning, use brushes to remove grease and dirt. Bearings should be handled carefully. Note that raceway surfaces may be damaged by foreign matter if bearings are rotated in cleaning oil.
• During finish cleaning, clean bearings carefully by rotating them slowly in cleaning oil.

In general, a neutral, water-free light oil or kerosene is used to clean bearings; a warm alkali solution can also be used if necessary. In any case, it is essential to keep oil clean by filtering it prior to cleaning.

Apply anti-corrosion oil or rust preventive grease on bearings immediately after cleaning.

17.2      Inspection and analysis

Before determining that dismounted bearings will be reused, the accuracy of their dimensions and running, internal clearance, fitting surfaces, raceways, rolling contact surfaces, cages, and seals must be carefully examined so as to confirm that no abnormality is present. It is desirable for skilled persons who have sufficient knowledge of bearings to make decisions on the reuse of bearings. Criteria for reuse differ according to the performance and importance of machines and inspection frequency. If the following defects are found, replace the bearing with a new one.

• Cracks and chips in bearing components.
• Flaking on the raceway surfaces and the rolling contact surfaces.

Three areas to look at when looking at bushes

• Control arm bushings are located between the control arm and the frame of the vehicle. They are essential for tyre alignment and directly affect tyre wear, handling, and vehicle control. Most vehicles use one or two control arms per wheel on both the front and rear suspension, with each arm housing two bushings. Many front-wheel-drive vehicles employ only the use of a lower control arm, whereas most bakkies and SUVs have both an upper and lower control arm.
• Stabiliser bar bushings are mounted underneath the vehicle on anti-roll bars, which prevent excessive vehicle roll (from side to side), especially during cornering. As a result, they are constantly twisting and flexing and can wear more rapidly. Usually, there are two stabiliser bars per vehicle (front and rear), each with two stabiliser bar bushings.
• Leaf spring bushings are another kind of bushing located in the ‘eyes’ of the leaf spring. Leaf springs are typically used in the rear suspension of heavy-load-carrying vehicles such as bakkies. The leaf springs support the weight of the vehicle and control the ride height while keeping the tyres in contact with the road. The load on leaf spring bushings is usually very high due to the weight and forces that bakkies are subjected to, and they need to be changed regularly.

18.       Replacing vehicle bushes

For an easy installation of bushing SE-SB-0104, there are the 3 steps to follow:

Indicate an auxiliary mark; this must be 90° to the rubber reinforcement. After installation, the rubber reinforcement should always be in the direction of the support (center of the vehicle).

When inserting the bush into the support, the auxiliary mark should be aligned with the marking on the housing. The rubber reinforcement should be pointing toward the direction of the support.

After the wishbone bushes are mounted, the other parts should be installed following the vehicle manufacturer’s recommended torque values.

18.1      When to change the bushes?

A vehicle bush system needs surveillance, and when symptoms of expiration appear, you should be prepared. Here are 4 signs of bad bushes that can help you:

• The steering wheel has a different position after cornering.
• Tyre wear.
• Excessive road noise and vibration.
• Looseness when handling the vehicle.

Key points to consider when replacing a bush:

• It goes without saying that you should always fit the correct bush for the vehicle – although, on the surface, they may look the same, there are many internal features designed for the specific application.
• In the case of anti-roll bar bushes, it is advisable to replace them in pairs.
• Always carry out a dimensional check of the part before fitting.
• Remember to torque-tighten the fixings with the vehicle on the ground. Failure to do so could put unnecessary stress on the bush in the wrong position, resulting in premature failure.
• Some bushes can be challenging to replace and may need specialist pressing tools. In this instance, it can be more economical to replace the entire component rather than just the bush.

19.       General instructions

Since rolling bearings are more precisely made than other machine parts, careful handling is necessary.

• Keep bearings and the operating environment clean.
  • In shipping bearings, since they are covered with proper anti-corrosion oil and are wrapped in antitarnish paper, the quality of the bearings is guaranteed as long as the wrapping paper is not damaged.
• Handle carefully.
• Bearings can be cracked and brinelled easily by strong impact if handled roughly.
• Handle using the proper tools.
• Keep bearings well protected from rust.
• Do not handle bearings in high humidity.
• Operators should wear gloves in order not to soil bearings with sweat from their hands.
• Experienced or well-trained operators should handle bearings.
• Set bearing operation standards and follow them.
• Storage of bearings – If bearings are to be stored for a long time, it is advisable that the bearings be stored on shelves set higher than 30cm from the floor, at a humidity less than 65%, and a temperature around 20℃. Avoid storage in places exposed directly to the sun’s rays or placing boxes of bearings against cold walls.
• Cleaning of bearings and their adjoining parts.
• Inspection of dimensions of adjoining parts and finish conditions.
• Mounting.
• Inspection after mounting.
• Dismounting.
• Maintenance and inspection (periodic inspection).
• Replenishment of lubricants.

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