PRELOADING

Description

Preload is defined as the internal load existing in any device before an external working load is applied. The application of preload to ball bearings is utilized to increase the ball bearing unit’s rigidity; to guarantee intimate contact between the rolling elements and the races at all times, thus insuring complete utilization of the actual running accuracies of the bearings; to elimi­nate the possibility of looseness from all phases of bear­ing operation; and to obtain the added effects on the bearing’s basic geometry such as a change in contact angle, a change in strain pattern or a reduction of the minute inaccuracies caused by surface irregularities.

Selection

The selection of the type of bearing and the mount­ing arrangement to be used is a function of the operat­ing conditions and requirements of the assembly. A realistic evaluation of these conditions and require­ments must be made in order to establish the design parameters. Often these parameters are contradictory in practice; therefore, the designer must strive for the optimum design through compromise. Design consider­ations, such as speed range, operating loads, rigidity requirements, accuracy desired, and ambient and oper­ating temperatures, determine which type bearing must be used, which type mounting and lubrication is re­quired and whether or not preload is necessary. These factors also dictatethe magnitude of the preload required and the method of its application.

If the design dictates that preload is required, a proper level must then be chosen. It should be stressed that while preload on bearings increases their rigidity it also reduces their life expectancy and increases their frictional drag. Therefore, the choice of the preload level to be incorporated must be made with care. Too often, however, the basic design parameters are contradictory and a compromise preload must be selected.

The Fafnir Bearing Company offers three standard preload levels for the various bearing types and sizes. These preload levels were established to satisfy most application requirements.

In the determination of limiting operational speeds corresponding to the permissible ball bearing preloads for machine tool spindles, many influencing factors are involved. Among those considered are spindle mass and construction; type of mounting; spindle rigidity; accu­racy requirements; spindle loads; service life; type of service, intermittent or continuous; and method of lubrication.

Because of the complexity of this subject, encom­passing all of the factors is difficult. Over a period of several years, data from actual field applications has been accumulated and compiled.   Extra precision ball bearings of the three basic series are included. In certain applications, such as the high-speed motorized spindles, specially preloaded extra-precision ball bearings are required. These bearings are zero preloaded (desig­nated FS-223). This means that the faces of the inner and outer rings are flush ground under negligible load.

Width Tolerances

The width tolerance for individual inner and outer rings is +.000 to —.005, but to allow for face grinding for various preload levels the total width tolerances for duplex pairs of bearings are as follows:

If additional bearings are involved, the total width tolerance is in proportion to the number of bearings.

SHAFT AND HOUSING DESIGN

Shafts

Shafts  are generally made from steel, hardened and ground all over. During the  design of a spindle or shaft it is highly desirable to plan the design  so that the shafts can be ground all over from one setting as a final opera­tion to assure true balance and running accuracy.  This is a basic requirement  for extreme accuracy or high­speed operation.

Housings

Housings  are generally made of cast iron and usually are seasoned to lessen possible distortions. The bore of the housing should be ground or precision bored and checked  at a number of points throughout its length and diameter to assure that it  is uniformly round and not tapered. It is preferable to mount the bearings in one casting. In this way the two or more housing bores required can be machined during one setting, thereby, helping to assure accurate alignment.

In many applications, a quill-type design is required in order to obtain spindle adjustment. This type of assembly can be advantageous,
in certain applications, in that it allows assembly of the bearings on the shaft and insertion of the bearings and shaft into a housing as a
bench assembly. Quill-type designs are also used when the main frame is made of materials having low hardness such as aluminum.

Where screws are used to fasten parts into the main housing, adequate section should be left under the screw holes to prevent
distortion of the housing bore when the screws are secured and the covers or other parts are pulled tightly into place.

Bearing Spacers

Spacers mounted between units of a pair of bear­ings are preferably made of steel, hardened and ground and should be sturdy in cross-section. The length should be produced by grinding the inner ring spacer and outer ring spacer simultaneously. It is important that faces of the spacers are square and that their parallelism is the best possible. All corners should be rounded to remove sharp edges and burrs.

The inside diameter of the inner ring spacer should clear the shaft, but not be so loose as to make it possible to mount and run them
eccentrically. For short spacers and high operating speed a clearance of less than .001″ over the maximum shaft diameter is generally satisfactory. For long spacers and low speeds this clear­ance may be increased to prevent the shaft from dis­turbing the face parallelism of the spacer.

The outside diameter of the outer ring spacer should be about .001″ less than the minimum bore of the housing. The overall design of the inner ring and outer ring spacers should be made so that a minimum of dead air space is left in the cavity between the bearings.

Housing Seals

A labyrinth combination of slinger and end cover provides a highly effective seal against intrusion of foreign material. This seal
design is recommended for use over a wide range of speeds. For exceptional slow speed applications a combination of slinger and com­mercial contact type seal is usually employed.

Slingers should be completely machined to assure true running. Their diameters should be concentric with the bore. The outside diameter of the slinger is often tapered, to throw off cutting compounds, coolants, etc., from the point at which such liquids may enter the spindle. A drip or runout groove adjacent to the open lip of the end cover is highly desirable and practical.

The axial clearances of the internal faces between slinger and end cover should be about 1/16 of an inch. The first radial clearance opening on any design through which liquid may pass should be made very close, approximately .003/.005″ on a side. The inner radial clearance should be between .0075″ and .015″. These figures are based on successful practice.

In sophisticated oil-air-mist lubrication systems it has been found that exhaust porting must be supplied between the bearings and
labyrinth type seals in order to insure adequate lubrication of the bearings and suffi­cient air flow for cooling.

SHAFT AND HOUSING FITS

Shaft Fits

The main purpose of the shaft fit is to assure proper security of the inner ring to the shaft. Under normal conditions of shaft rotation, a loosely fitted inner ring will creep on the shaft leading to wear and peening. This condition will be further aggravated by increase of load or speed. To prevent creeping or slipping, the inner ring should be mounted firmly in place and held securely against the shaft shoulder. However, it is important that the shaft diameter should not be so great an inter­ference fit as to cause undue radial preloading of the bearing. Such a condition could lead to excessive heat generation and increased power consumption.

As a general rule it is recommended that the shaft size and tolerance for seating extra-precision bearings be the same as the bearing bore.
In the case of pre‑ loaded bearings, the ideal shaft fit is a line-to-line fit since an excessively tight fit expands the bearing inner ring and
increases the bearing preload which can lead to overheating. In the case of extremely high-speed applications, however, a minimum press fit of the inner ring is required to insure that the proper fit will be maintained at operating speed. This press fit must be sufficient to compensate for expansion of inner ring due to centrifugal force. Any creeping or shifting of the inner ring on a high-speed device would tend to destroy the balance of the shaft assembly.

The extent of preload change due to an interference fit on the shaft can best be illustrated by the following example: A duplex pair of 2MM9111WI CR DUL bear­ings, with 35 pounds built-in preload, mounted on a shaft that provides an interference fit of .0004 inches will
have a mounted preload level of approximately 180 pounds. This significant increase in preload will result in elevated operating
temperatures.

Housing Fits

Under normal conditions of rotating shaft, the outer ring is stationary and should be mounted in the housing with a hand push or a light tap fit. Should the housing be the rotating member, the same fundamental considera­tions apply in mounting the outer ring that were used for the inner ring mounted on a rotating shaft.

As a general rule for rotating shaft applications, the minimum housing bore dimension for extra-precision bearings should be established
as the maximum bear­ing outside diameter. The tolerance of the housing bore should be the same as the tolerance for the outer ring.

Numerous designs utilize the fixed and floating principle. The previously mentioned fitting practice should be utilized on the fixed
end only. In order to insure that the opposite end will float, and to corn pen-sate for thermal changes within the device, it is neces­sary to supply a looser housing fit. Generally this fit will be sufficient if the average fit is made .0002″ looser than the average fit of the fixed end. If a heat source is present, such as an integral motor, added looseness in the housing will be required.

BALL BEARING TYPES

Non-Filling Slot Type

M-K and MM-K type ball bearings are designed to carry radial, axial or combined loading. Because of the fully shouldered inner and outer races, these bearings can carry axial load in both directions and have relatively high-speed capability.

Annular Contact Type

2M-WI and 2MM-WI types, with 12° contact angles, maximum complement of balls, and one-piece inner ring-piloted retainers are designed to meet the needs of machine builders for extra-precision bearings which will operate with minimum temperature rise for a wide range of speeds and operating loads. In order for machines to produce more accurate work at a higher production rate, the bearings must provide a high degree of rigidity in both axial and radial directions. For example, on precision turning machines, cutting tools impose heavier loads on bearings than those en­countered in precision grinding. In the former, speeds are slower and loads heavier than in the latter, where speeds are high and loads light. 2M-WI and 2MM-WI types give machine builders the flexibility required to meet such variations in application requirements

3M-WI and 3MM-WI types, manufactured with 25° contact angles, maximum complement of balls, and one-piece inner ring pilot retainers are designed primarily for use on applications where the loading on the bear­ings is predominately thrust and a high degree of axial rigidity is a definite requirement, or when a closer balance between the axial and radial bearing spring constants is required. Typical applications for these bearings are large, vertical, rotary-surface-grinders, horizontal and vertical disc-grinders, heavy-duty lathes, boring machines and milling machines.

2MM-WO types, with 18° contact angles, are de­signed for extremely high-speed applications where the centrifugal force of the balls must be considered. Unlike the MM-WI type which has counterbored outer rings and inner ring land piloted retainers, the 2MM-WO type has full shoulders on both sides of the outer race and a low land on one side of the inner ring. This design permits assembly of a maximum complement of balls and a one-piece channeled retainer piloting on the pre­cision ground lands of the outer ring.

Ball Screw Support Bearings

To meet the requirements of the highly sophisti­cated numerically controlled machinery field, The Fafnir Bearing Company has developed a new family of ball bearings especially designed for ball screw applications. The design criteria for these bearings are maximum axial rigidity, low drag torque and extreme control of lateral eccentricities. These bearings have been used successfully on all types of tape controlled machinery, precision instruments, missiles, and slide actuators or in-feed mechanisms used on standard machine tools.

The Timken name stands for high quality and outstanding performance.  Using their capabilities in bearing technology, manufacturing, engineering support and distribution, they provide their customers with smart, cost-effective friction management and power transmission solutions that help outperform the competition.

Expanded Housed Unit Offering

From the manufacturer that brought you reliable Fafnir^® ball bearing housed units and Timken® SAF® pillow blocks comes a growing line of roller housed units. We’ve expanded our offering to include Timken® Type E housed units featuring our own advanced tapered roller bearings and seals.

The latest innovation is their  new series of Timken^®

Type E roller housed units. Timken engineers have applied more than 110 years of expertise in bearing design — and leadership in tapered roller bearings — to bring you an optimized performance tapered roller bearing housed unit. They have improved thier own design to create the new standard in performance.

The Timken brand stands for outstanding quality in everything they do, from product design and manufacturing to engineering support and distribution.

Timken’s commitment to quality, service and delivery adds up to an unbeatable value for customers everywhere.

As the only premium bearing manufacturer to also make super-clean, high-alloy bearing steel, Timken knows how critical quality materials are to product performance.

They have also implement the Timken Quality Management System in every plant worldwide, so each bearing product meets the same high performance standards — no matter where in the world it is manufactured.

Dependable Service

Their roller housed units are backed with the service of Timken’s industry-leading experts who are ready to assist you with product design, application knowledge and 24/7 field engineering support — anything you need to help improve uptime and maximize equipment performance.

On Time Delivery

Timken Bearings know short lead times are critical to keeping your operations up and running. Many of their Type E roller housed units are in stock for immediate shipping. For special orders, They work quickly to manufacture and deliver your products on time.

Timken 204RR6 Lawn mower deck bearings

 .. Still can be bought for $10.00 each (plus shipping and handling)

and

NSK 6205DDUC3 Electric Motor Quality Bearing

…. Still can be purchased for $6.12 each .. (plus shipping and handling)

Check them out NOW  .. they have been extended until July 31st, 2011 ..

HOT ROLLING

 ANNEALING

Annealing is heating uniformly to a temperature within or above the critical range and cooling at a controlled rate to a temperature under the critical range. This treatment is used to produce a definite microstructure, usually one designed for best machinability. It is also used to remove stresses; induce softness; and alter ductility, toughness or other mechanical properties.

HOT BED COOLING

This is the standard process of air cooling the tubing immediately following piercing on a special table or hot bed that continuously advances the tubing as additional tubes are finished. Tubing that is hot bed cooled may require additional thermal treatment to obtain optimum machinability or required mechanical properties.

MILL ANNEALING

Mill annealing is a controlled cooling of the tubing immediately following piercing which produces – in some low and medium carbon alloy grades – the optimum hardness and microstructure for machinability.

NORMALIZING

In this process, steel is heated uniformly to a temperature at least 99° F (37° C) above the critical range and cooling in air to room temperature. This treatment produces a recrystallization and refinement of the grain structure and gives the product uniformity in hardness and structure.

PROCESS TEMPERING

Process tempering is performed to improve subsequent fabrication of the material (machining, cold working, etc.). See Tempering.

QUENCHING (QUENCH HARDENING)

This processes heats steel uniformly to a temperature above the critical range and cools it rapidly in a liquid medium.

SPHEROIDIZE ANNEALING

This special type of annealing requires an extremely long cycle. Spheroidize annealing is used to produce a globular condition of the carbide and maximum softness for best machinability in some analyses or to maximize cold formability.

STRESS FREE

This designates tubing that has been hot rotary straightened. Minimal straightening stresses develop in hot rotary straightening, so stress relief tempering is not needed. Cold rotary straightening and stress relieving can substitute for hot rotary straightening.

STRESS RELIEVING

Stress relieving is a final thermal treatment used when stress-free material is desired. Its purpose is to restore elastic properties and minimize distortion on subsequent machining or hardening operations. This treatment is usually applied on material that has been heat treated (quenched and tempered). Normal practice is to heat to a temperature 99° F (37° C) lower than the tempering temperature used to establish mechanical properties and hardness.

TEMPERING

Tempering heats uniformly to a temperature under the critical range, holding at that temperature for a designated period of time and cooling in air. This treatment is used to produce one or more of the following end results:

• To soften material for later machining or cold working. Also referred to as a Process Temper.
• To improve ductility and relieve stresses resulting from prior treatment or cold working.
• To produce the desired mechanical properties or structure in the second step of a double

COMPOSITION CAGES (CR)

Composition cages combine light weight, precision and oil-absorbing features which are particularly desirable for use on high speed applications. This (CR) cage is a ring-piloted type and is available in Timken’s patented channeled construction for very high-speed applications. It is particularly associated with the outer-ring piloted, extra precision WO series bearings

SPECIAL CAGES

For certain very high contact angle, light section aircraft bearings, molded nylon “snake” cages are employed, Cages are also made with high temperature materials in the various configurations described previously.

IRON SILICON BRONZE CAGE (SMBR) AND MACHINED STEEL CAGE (MSR)

The SMBR and MSR cages are ring-piloted. The advantages of these cages are high strength even at elevated temperatures as well as high-speed capability due to the ring-piloted construction. In many cases these cages are silver plated for use in applications requiring high reliability.They are available in both ball and roller bearings.

BRONZE CAGES

Bronze Cages are generally installed in bearings which are designed for use on heavily loaded applications, such as, deep well pumps, woodworking machinery, and heavy construction machinery. The following types of bronze cages are available:

CAST BRONZE CAGE (BR) This cage, a ball-piloted bronze retainer designated by the letters BR in the Fafnir/Timken line, utilizes two identical halves which are riveted together.

MACHINED BRONZE CAGE (MBR) These cages are machines all over to provide ring riding surfaces and good static and dynamic balance. They are commonly incorporated as inner ring piloted designs in the 7000 angular contact series. Because of their superior strength, these cages are generally used on heavily loaded applications such as deep well pumps, woodworking machines, and heavy construction machinery.

Cages (sometimes referred to as rolling element separators or retainers) perform an important function in the proper operation of rolling bearings. They serve to maintain uniform rolling element spacing in the races of the inner and outer rings of the bearings as the rolling elements pass in and out of load zones. Cage types in several materials and configurations have been developed to meet various service requirements.

Some of the materials from which cages are made include pressed-steel, pressed-bronze, cast bronze, machined bronze, machined steel and compositions of various synthetic materials.

STEEL CAGES

Steel cages are generally ball-piloted and are available in the following types:

PRESSED-STEEL FINGER TYPE CAGES (SR) Light in weight and made from strong, cold rolled steel, the pressed-steel cage because of its compactness is the optimum design for use in shielded and sealed bearings which must conform to the ABEC boundary dimensions. This is a general purpose design and is frequently used for ABEC-1 radial ball bearing sizes.

WELDED STEEL CAGES (WR) The welded steel cage provides greater strength, increased rigidity, and better pocket alignment than the finger type. The projection welding of the cage halves eliminates weakening notches or holes and fingers or rivets. It assures better mating of cage halves circumferentially and radially. This construction also provides more uniformity of ball to pocket clearance. Improved pocket geometry permits higher speeds, reduces cage wear, provides cooler operation, and improves and extends lubricant life. This cage is standard in most radial non-filling slot bearings of the open, shielded, and sealed types.

….. until June 30, 2011. 

Timken brand lawn mower deck bearings, reference number 204RR6 A4362 are known as Farm Radial Specials.  The bearing dimensions are as follows:  .75 ID x 1.7805 OD and .610 width.

These bearings are a heavy duty radial ball bearing designed for use in the spindle housing of commercial and riding lawn mowers, commonly used in applications of John Deere Mower Decks of 38”, 42”, 46”, 48”, 54” and 60”.

Our Timken Lawn Mower Deck Bearings include trash guard double rubber seals.  These are standard on this bearing to insure protection from contaminants of soil, grass and moisture.  Each trash guard seal consists of carbon steel in which a Buna N Seal is attached.  The seals are crimped into the outer ring with a rubber lip riding on the inner ring.  Slight misalignment of the bearing will not affect its effectiveness.

The bearing is factory lubricated with grease.  The prelubrication and double seals eliminate the need for extra servicing.

The nylon retainer’s shock resistance and longevity are a match for the rough conditions and maintenance demands of most applications.

This bearing can be found here … for $10.00 each.