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Technical Information

Lubricants

    The lubrication of miniature and instrument bearings has four essential aims:
  • to minimize wear of the functional surfaces
  • to reduce the torque of the bearing
  • to facilitate cooling by dissipating heat produced by rotation
  • to protect the bearing from corrosion

    • The choice of a lubricant is most important because to a large extent the performance and working life of the bearing will depend on this. In choosing our lubricants, the following points in particular has been considered.

  • the driving torque available
  • the load to which the bearing is subjected
  • the rotational speeds of outer and inner races
  • the ambient temperature
  • the heat produced by rotation of the bearing
  • the normal operating condition, e.g. intermittent, continuous duty, etc.
  • the admissible noise level
  • the expected life of the bearing
  • storage life of bearing before use

Amount of lubricant

An excess of lubricant is always undesirable. In open bearings the surplus lubricant is ejected by centrifugal force and deposited on adjacent parts, which is undesirable. In a closed bearing the surplus lubricant prevents the dissipation of heat, increases resistance and consequently, the risk of over-heating.
The amount of lubricant to be used in a bearing should be decided with reference to its operating conditions. The smaller the amount of lubricant introduced into a bearing, the more important it is that it should be placed in the critical places, which are the cage pockets and the ball tracks.

CAGES (RETAINERS)

The most common types of cages (also called retainers) used for miniature ball bearings are the TWO PIECE STEEL RIBBON and the ONE PIECE STEEL CROWN type cages. The crown type is better suited to low torque , low speed applications. These two types of cage are interchangeable in the majority of applications. These cages are usually made out of a variety of grades of stainless steel depending upon the manufacturer. Grades usually used are AISI302, AISI410 or AISI430.

Larger bearings (over 10mm bore size) usually utilize a TWO PIECE STEEL RIVETED cage allowing for higher loads, and more vibrational and acceleration stresses to be placed on the bearings.

High speed applications require bearings with a crown type retainer manufactured from various types of synthetic materials including NYLON, DELRIN AND PHENOLIC LAMINATES. Please note however that the term "high speed" is relative to the bearing size. i.e. What is "high speed" for a 10mm bearing may not be "high speed" for a 4mm bearing. Other cage types are available, but are beyond the scope of this document.

CLEANLINESS

Strict cleanliness is an essential condition for the good operation of all miniature bearings.

Some factors affecting the quality of bearings are:
  • full temperature control and air filtration during assembly
  • ultrasonic cleaning of all components after different stages of manufacture
  • cleaning of all component parts before assembly
  • the assembly of parts in special CLEAN ROOMS, in which the temperature, humidity and dust content of the atmosphere are controlled to precise limits

MATERIALS

Standard materials used for the races of our miniature ball bearings are AISI440C STAINLESS STEEL and SAE52100 CHROME ALLOY BEARING STEEL.

AISI440C Stainless Steel bearings are heat treated to a hardness usually from Rockwell C 58 to Rockwell C 61+. The maximum recommended operating temperature is 177 Celsius (350 Farenheight).

SAE52100 Chrome Steel Bearings are usually slightly harder than AISI440C bearings (Rockwell C 61-64). The maximum recommended operating temperature of chrome steel bearings is 120 Celsius (248 Farenheight).

 

 

NOISE LEVEL

The noise level of bearings depends essentially on the following factors
  • the geometric precision and quality of the tracks and balls
  • the cleanliness of the bearings and the lubricant
  • the precision with which the play is taken up when mounting the bearings
  • the value and direction of loads
  • the rotation speed

PROBLEM SOLVING

PROBLEM CAUSE REMEDY
Noise High pitched metallic Poor lubrication Improve lubrication
Clearance too small Correct clearance
Poor fitting Investigate mounting method and seating
Excessive load Examine shaft and housing tolerances for closing effect
Low pitched metallic Brinelled raceway Avoid shock loads
Regular Rust or damage Check & replace seals & relubricate  
Flaking of raceway surface Improve lubrication & check fitting, clearance and fixing method
Irregular Ingress of foreign matter Check & replace seals and relubricate
Excessive clearance Correct clearance
Damage & flaking of rolling element Reduce loads and/or clearance
Variable Varying clearance due to temperature changes Check fits taking housing material and temperature into consideration
Damage to raceways Improve lubrication & check fitting, clearance and fixing method
Heavy vibration Flaking of raceway and rolling element Improve lubrication & check fitting, clearance and fixing method
Ingress of foreign matter Check & replace seals and relubricate
Excessive clearance Correct clearance
Poor location Ensure abutment face and fitting diameter are perpendicular
Excessive heat generation Clearance too small Correct clearance
Poor location Ensure abutment face and fitting diameter are perpendicular
Excessive load Examine shaft and housing tolerances for closing effect
Poor lubrication Improve lubrication
Creep Maintain recommended shaft & housing fits Lubrication breakdown Too much grease Use correct lubricant quantity
Ingress of foreign matter Check & replace seals and relubricate

SEALS & SHIELDS

Seals and shields are situated between the inner and outer rings of the bearing. The purpose of seals and shields is to protect the inside of the bearing from contaminants and to help prevent the leakage of lubricant from the bearing.

Shields - Made from STEEL or STAINLESS STEEL. Shields do not contact the inner ring of the bearing and therefore do not increase the running torque. Shields can be either crimped into the inside of the outer race (type ZZ) or held in the outer race by circlips (type ZZS). The ZZS type are removable to allow inspection and maintenance of the bearing.

Seals - Usually made from NITRILE. Seals are either CONTACT (type 2RS) or NON CONTACT (type 2RU). Contact type seals which are secured in within the outer race and contact the inner race greatly increase start up and running torque, thereby reducing running speed. They also provide a very effective method of excluding contaminants and retaining the bearing lubricant. Standard Nitrile seals have a maximum recommended temperature of 100 Celsius (212 Farenheight). Some bearings are also available with PTFE seals offering very effective sealing whilst minimizing torque. Teflon seals can operate at temperatures up to 300 Celsius (572 Farenheight). Note however that whilst the seals can withstand these temperatures the bearing itself will need special treatment if it is to be used at any temperature greater than those shown in our materials section.


RADIAL CLEARANCE

The radial play of a bearing is equal to the total radial displacement, in the median plane perpendicular to the bearing axis of the inner ring in relation to the outer ring, under the effect of a small measuring force.

Because of the complexity of measurement and taking into consideration manufacturing tolerances, radial play is always indicated between two limits, usually 5 to 15 µm (.0002 to .0006").

Play is not a criterion of the quality of a bearing but if badly chosen or unfavorably influenced during mounting, it can adversely affect the operation and even reduce the working life of a bearing.

This table shows the basic radial clearances available for miniature bearings and applications for these clearances.
Dimensions shown in microns µm 0.001mm and (1/10th thou 0.0001").
ISO Code Description Minimum Maximum Applications
MC1 Tight 0 (0) 5 (2) Radially loaded low backlash gear systems. Very low speed.
MC2 3 (1) 8 (3)
MC3 Standard 5 (2) 10 (4) Tape Guides, Synchros, Servo Motors, Low Speed Electric Motors and Gear Trains. Gyro Gimbals (Horizontal Axis).
MC4 8 (3) 13 (5)
MC5 Loose 13 (5) 20 (8) High speed Electric Motors and Tape Guides. Provides some compensation for axial loading. Gyro Gimbals (Vertical Axis).


Calculation of the theoretical life expectancy of ball bearings



The theoretical life expectancy has no practical value unless the following conditions are scrupulously fulfilled:
- Strength and direction of constant loads carefully determined. See page 56
- Constant speed
- Constant temperature not exceeding 100 degC
- Strict cleanliness in mounting and during running
- Careful choice and dosage of lubricant
- Mounting strictly in accordance with the instructions given in pages 53 to 55
In all cases of complexity or doubt it is advisable to consult our technical staff.

For calculating the load capacity and the theoretical life of bearings we have used the formulae the theories based on those of IS0 and AFBMA standards.

1. Life expectancy of radial bearings and thrust bearings


Legend
L = Life expectancy in millions of revolutions
C = Dynamic load capacity in N
P = Equivalent dynamic load in N
C/P = Load ratio

2. Life expectancy in hours Lh



Legend
Lh = Life expectancy in hours
n = Revolution in rpm

3. Definitions

L, Lh = Number of millions of revolutions or hours at constant speed that 90% of a group of apparently identical bearings will attain or exceed before the first evidence of fatigue develops. The life which 40% of the group of ball bearings will complete or exceed is approximately five times this life expectancy.

C = Dynamic load rating. This is the constant radial load, stationary with respect to the outer ring, that a bearing can endure for a rating life of one million revolutions of the inner ring or 500 hours at 33 1/3 rpm.
The dynamic load takes into account:
- repeated deformation of several elements (tracks and balls) as a function of the mechanical resistance of their materials and of their geometric form
- frequency of loads
- an empirical probability factor

P = Equivalent dynamic load which takes into account the distribution of axial and radial forces affecting different elements as a function of their elasticity and of their geometric form (radial play, tracks, and ball diameters).

Co = This is the pure radial load which affects the bearing under the following conditions:
- zero rpm
- very slow oscillating movements
- very low revolutions
This load is permissible when, distributed between balls and tracks, a permanent deformation of 1/10,000 of the ball diameter is not exceeded.

Po = Equivalent static load.


Newton / Ib conversion

1 Newton = 0,225 Ib
1 Ib = 4,45 Newton