Tufnol CBM 2008 Technical information

TUFNOL materials for engineering

 

TUFNOL CBM 2008

Composite Bearing Material

 

TECHNICAL INFORMATION

 

TUFNOL CBM 2008 is a strong, rugged bearing material with excellent wear performance.  It can be used for a very wide variety of rotating or reciprocating bearings, in tough and demanding working conditions.   It is suitable for many applications with or without additional lubrication although, in the majority of cases, bearing performance will be greatly enhanced by the use of a suitable lubricant.

The high dimensional stability of the material ensures that swelling is negligible for most applications within the design and dimensional parameters set out in this data sheet.  Consequently it is possible to ignore the presence of rain water, high humidity, exposure to outdoor conditions in the 'splash zone' of a marine environment, even total immersion in water. 

Thus, the material can be used in a variety of fluids without special precautions including oils, greases, fresh water, salt water and many water-based process fluids, as long as they are free from abrasive particles.1

 

FRICTION AND WEAR.

 

In any sliding bearing, energy used to overcome friction is converted into heat, causing a temperature rise at the rubbing surface.  This heat is dissipated away through the bearing and housing or, in some cases, carried away by a flow of lubricant or coolant.  In continuous operation, a bearing achieves a stable running temperature when the rate of heat dissipation matches the rate of heat creation in the bearing.  Higher loads and speeds create heat more rapidly and, with non-metallic bearing materials, the maximum short-term operating conditions for the bearing are limited primarily by the need to maintain an acceptable operating temperature for the material.

For operation at higher loads and speeds, lubrication reduces friction, limiting the heat generated and keeping temperatures within the capabilities of the material. 

Lubrication also has a major influence on wear.  In practical applications, operating conditions are often governed by the need to limit the rate of wear, to give an acceptable working life. 

 

PV LIMITS

 

Actual bearing performance therefore is influenced by many factors, such as load, speed, coefficient of friction, surface finish and hardness of mating surfaces, lubrication and the heat dissipation characteristics of the design.  Calculations which attempt to take account of all these factors become extremely complex, particularly where lubricated bearings are concerned.  

For design purposes, a simplified method of evaluation is widely used, based on figures for 'PV' (pressure x velocity).  For this, the maximum limits of load and speed for satisfactory performance without lubrication are tested under laboratory conditions. From this data, judgements may then be made about the likely performance of a bearing under similar, or better, working conditions.

Recommended PV limits for TUFNOL CBM 2008 are shown in Figure 1. 

 

CALCULATING THE OPERATING PV

 

Bearing pressure.  In a cylindrical bearing, the pressure exerted between the shaft and bearing is not constant over the contact area.  However, in a well engineered cylindrical bearing with an accurately fitted shaft and housing, it is common practice to calculate the projected area of the bore, and to use this figure for area to assess the bearing pressure,

i.e. Bearing area = Bore diameter x Shaft length in contact

Where a bearing will be operated with an unusually large clearance or other factors which could lead to an excessively uneven distribution of the load, assessment of the working pressure should be adjusted accordingly.

 

Running speed.  The maximum sliding speed in a cylindrical bearing is calculated from:

Speed in m/min

          = 0.00314 x bore diameter (mm) x max revs per minute

 

Operating PV.  The PV conditions (bearing pressure x running speed) under which the bearing will operate are calculated and compared with the maximum recommended in Figure 1

 

Under some circumstances, higher loads may also be acceptable, although the wear rate is likely to increase.  Where necessary, please seek advice from our Product Engineering Department.

 

LUBRICATION

 

Grade CBM 2008 is self lubricating in lightly loaded applications.  However, friction will be lowered and performance enhanced by the use of a suitable lubricant.  

Lubrication used in various applications ranges from a slight smear of grease on assembly, up to fully designed circulating systems with copious supplies of filtered lubricant pumped under pressure.

Most common lubricating oils and greases are suitable for use with TUFNOL.  Water is also an excellent lubricant as are many water based fluids, providing they are not chemically corrosive and do not contain abrasive solids.

Other process fluids such as inks, dyes, cleaning fluids, metal treatment solutions, effluents, and coolants may be allowed to enter the bearing, to provide lubrication and eliminate the need to fit seals.1

Where necessary, to prevent excessive wear, arrangements should be made to exclude dirt or abrasive matter from the rubbing surfaces.  For example, where a bearing is lubricated with river or sea water, any sand or silt should be filtered out before the water enters the bearing.

Grooves are often used to help distribute lubricant, and these can also provide an escape route for any dirt or debris which may otherwise accumulate.  Simple shapes of grooves, giving even distribution of lubricant, are generally preferred. Obviously, grooves should not be placed in the most heavily loaded areas of the bearing, and should not interfere with the beneficial build up of lubricant pressure between bearing, and shaft under hydrodynamic conditions.

 

SPECIAL TREATMENTS AVAILABLE

 

Oil stabilising.   If required, material can also be heat treated in oil, to further improve its thermal stability at elevated temperatures.  This treatment is normally carried out prior to machining.

 

Lubricant pre-coat.   Where required, finished components can be supplied with a variety of pre-treatments, including spray coatings of dry lubricants or greases, ready for assembly.  Please discuss your requirements with Tufnol Composites Ltd

 

MATING SURFACES

 

For optimum performance, a hard smooth mating surface is preferred.  Surface finishes in the range of 0.4 to 0.8µm CLA are commonly accepted and mating materials such as hardened steel, hard chrome plated or stainless steels or gun metal are often used.  Surface hardness better than Rockwell C50 is usually considered ideal but TUFNOL laminates are reasonably tolerant and softer shaft materials have given good service in a great many bearings where the loads and speeds were not exceptionally high.

In some cases, mating materials made from other plastics have been used.  Whilst this may be acceptable for special purposes, heat dissipation from the rubbing surfaces is restricted and it is not an ideal arrangement.

 

BEARING DESIGN - GENERAL PRINCIPLES

 

Design of CBM 2008 bearings follows conventional engineering practice and, in many cases, CBM 2008 can be used as a direct replacement for another material.

When designing from scratch, the following guidelines may assist.

A length to shaft diameter ratio of 1:1 is generally considered ideal because long bearings (over 2:1) can contribute to the build-up of heat. Allowance should be made for the dimensional movement which can take place due to changes in the temperature of the material (see Figure 7).

The housing should fully support the bearing in the correct alignment and prevent unwanted axial or rotational movement of the bearing within the housing.

Typical wall thicknesses for CBM 2008 bearings are shown in Figure 4.    To assist heat dissipation, excessive wall thicknesses are to be avoided.  However, where high shock loads are expected, thin wall thicknesses are usually avoided.

To avoid weakening the wall, as a general rule any lubricant grooves should not exceed one third of the wall thickness.

All laminated plastics are able to absorb a small amount of water and, when they do, slight dimensional changes take place.  However, the synthetic fabric from which CBM 2008 is made ensures that dimensional movement due to absorption is negligible for most applications within the design and dimensional parameters set out here (i.e. normal wall thicknesses and normal running clearances). 

This makes the material very easy to use in outdoor conditions and many other working environments.

Recommended running clearances are shown in Figure 8.

 

FIXING METHODS.

Press fitting is the most commonly used fixing method for small cylindrical bearings.  However, when temperature fluctuations are likely to occur, some form of positive mechanical fixing is preferred. Also, where a cylindrical bearing is required with minimum initial clearance, positive fixing is recommended, as this improves dimensional control without in-situ machining. Typical fixing methods include keys, set screws, pegs, flanges with screws or adhesive bonding.  However, it is important not to use methods such as grub screws, rivets or self tapping screws, which can distort the shape of the bearing and cause binding on the shaft.

 

Flanged bushes.

Flanged bushes can be used with bolts or countersunk screws and the flanges can be produced in several ways.  If the required flange diameter is a little greater than the outside diameter of the bush itself, it is usual to produce the flanged bush in one piece from oversized material. However, for greater flange diameters, fabricated components are generally preferred. In such cases, the flange is produced from TUFNOL sheet material and a step is machined on the outside diameter of the bush (see Figure 3).  The flange is then bonded on to the end of the bush, using a high strength epoxy adhesive.  In the majority of cases, this is perfectly satisfactory, but where very arduous duty is anticipated, an even stronger flange joint can be achieved by machining screw threads in both TUFNOL components and screwing the flange on to the bush after coating the threads with epoxy adhesive.  Flanges produced from sheet material offer maximum compressive strength, in view of the more suitable direction of their laminar structure and bushes with fabricated flanges are therefore very successful.

 

Press fitting. 

When a thin walled bearing is interference-fitted into a housing, an allowance must be made on the bore to allow for it to close in by an amount equal to the interference.  On bearings with thicker walls, the material wall thickness will compress slightly and a smaller allowance can be made (see Figure 4).  When calculating bore closure, it is important to take the maximum interference which could occur within the machining tolerances.

The bush or housing should be made with a lead chamfer to avoid damage to the bush while it is being pressed in.  In fitting, the bush should be carefully aligned with the axis of the housing and pressed or pulled into place with steady force.

To achieve closest possible tolerances with press fitting, bearings can be machined or reamed to final size after fitting.

The use of key strips can allow bushes to be lightly press fitted, as they will prevent unwanted rotation of the bearing within the housing

Split bearings and half bushes

Split bearings should be firmly retained and supported within the housing.  Key strips or keep plates are commonly fixed to the housing using countersunk screw.  Such fixings should key with the bearing for at least half of its wall thickness.  

 Adhesive bonding.

Where adhesive bonding is used, it is important to select an adhesive which is suitable for the working conditions of the bearing, especially the temperature.  Many adhesives are compatible with TUFNOL, but the adhesive must also be suitable for bonding to the metal housing or support. Adhesive manufacturers’ recommendations should be carefully followed.

Adhesives may also be used in addition to mechanical location or fixing.

Slideways and wear strips. 

Slideways should be fully supported and long slideway should be bonded or fixed at intervals along their length.  Wear strips would typically be held every 100 to 150mm along their length by screws or rivets with heads recessed well below the surface.  Edge retaining strips are also commonly used.  See also adhesive bonding below.

 

 

BEARING CALCULATIONS

Where TUFNOL CBM 2008 is replacing an existing bearing, the basic dimensions of shaft and housing will usually be fixed.  TUFNOL bearings can range in size from very small machined items to very large bearings fabricated from a number of parts.  CBM 2008 tubes are made up to 660mm (26") outside diameter and, in addition to bearings they can also be used for items such as split wear rings, piston rings, and other similar applications, where good toughness and wear resistance is required.

Calculation of bearing sizes should follow the method shown in the following example to ensure that the clearance in the bore is correctly designed.

 

Instructions for calculating bearing dimensions.

 

Data required:

(i)   Proposed housing diameter and tolerance.

(ii)  Proposed nominal wall thicknesses (see Figure 4 if required).

(iii)  Proposed shaft diameter and tolerance.

(iv)  Proposed bearing length.

 

Compile and calculate the following:

Max housing diameter

 

 

= A

Interference fit (if required) (Figure 5)

 

 

= B

Min. bearing outside diameter

 

A+B

= C

Machining tolerance on bearing outside diameter (Figure 6)

 

 

= D

Max bearing outside diameter

 

C+D

= E

Minimum housing diameter

 

 

= F

Maximum interference

 

E-F

= G

Expected bore closure % (Figure 4)

 

 

= H

Maximum actual bore closure

 

H% of  G

= J

Thermal expansion allowance (Figure 7)

 

 

= L

Minimum final running clearance (Figure 8)

 

 

= M

Maximum shaft diameter

 

 

= N

Min. bearing inside diameter

 

N+M+L+J

= P

Machining tolerance on bearing inside diameter (Fig. 6)

 

 

= Q

Maximum bearing inside diameter

P+Q

= R

 

Note that the variation in bore clearance can be quite substantial when an interference fit is used, as it arises from the sum of the machining tolerances on the shaft, housing and the inside and outside of the bearing. As a result, this type of fit often requires machining tolerances to be the minimum practicable. Some sizes of bush can be machined by a method which allows for an accurate tolerance on wall thickness to be maintained. However, where a minimum variation in clearance is required, bearings are bored after fitting into the housing.

 

    

 

 

 

TYPICAL APPLICATIONS FOR TUFNOL CBM 2008.

 

Agricultural 

Bearings for pig meal mixers and other equipment.

Kicker shaft bearings for fertiliser distributors.

Sluice gate bearings.

Vacuum pump vanes.

Automotive

Slide bearings and tubes for forklift trucks.

Brewing and bottling

Cask washer bearings.

Pump bearings.

Building and Construction

Lift guides and shoes.

Chair lift top pivot bushes.

Expansion bearings for both steel and concrete members.

Chemicals and gas    

Bearings, bushes, and neck rings.

Bucket and piston rings.

Pump sleeves.

Pump stator bearings.

General engineering

Bearings.

Bucket rings.

Bushes.

Cams.

Machine tool sideways and wipers.

Neck rings.

Rotors.

Valves.

Blades/vanes for air tools.

Bandsaw guides.

Sliding shoes in sliding-shoe pumps for bilge, mine and oil traps.

Valve disks for water strainers.

Glass and pottery

Bearings and sleeves for clay mixing plant.

Metals and metal-finishing

Jockey roll bearings.

Rolling mill bearings and thrust rings.

Slideways.

Roll neck bearings in steel mills.

Mining and quarrying  

Bearings.

Submersible pump bearings.

Marine & Offshore engineering

Lock gate bushes

Lock gate hinge posts.

Sluice gate and valve facings.

Capstan bearings.

Culvert gate valve bearings.

Hydraulic cylinder ram eye bushes.

Lock gate heal posts.

Lock gate bogies.

“J” tube and riser support bearings.

Underwater bearing pads, skid beams and guides.

Water lubricated bearings.

Paper and printing

Bearings of many types.

Railways

Bearings.

Vacuum brake components.

Shipbuilding

Cargo door hinge bushes.

Loading ramp bushes.

Windscreen wiper bushes.

Davit sockets and bushes.

Fairlead bushes made.

Bearings and friction cones for tug boat fenders.

Stern shaft seal facings.

Rudder carrier bushes.

Rudder thrust bearings.

Slipper pads.

Cargo pump bucket rings.

Winch and capstan bearings.

Crane / davit pivot bushes.

Cargo winch and capstan bushes.

Mooring roller bushes and thrust washers.

Circulating pump bushes.

Stern roller bearings.

Slipway pads.

Sonar bushes and guide strips.

Folding hatch cover bushes and washers.

Sliding door and window rubbing strips.

Deep water pumps.

Anchor windlass bearings.

Transom flaps.

Blast door hinges.

Periscope bearings.

Submarine driving plane and actuating linkages.

Bow cap door hinges.

Scraper box rings.

Stabiliser bearings.

Stern tube bearings.

Submersible pump impellers, machined and fabricated.

Rudder Bearings in tubes or staves.

Water pump bearings and bucket rings.

Friction discs in submersible pumps.

Valve plates.

Textiles

Bearings for spindles on spinning frames.

 

 

Machined parts in CBM 2008

 

TUFNOL Composites Ltd have large, fully equipped machine shops for the production of bearings and components and we will be pleased to quote prices for finished items, made-to-order. 

 

We are registered to BS EN ISO 9001:2000 and we supply materials and components to many major manufacturing companies in Britain and throughout the world.

 

For those who wish to produce their own components, TUFNOL CBM 2008 can be readily machined using standard machine tools, using conventional machining techniques for plastics materials.

 

 

PHYSICAL PROPERTIES of TUFNOL CBM 2008

 

SHEET 

PROPERTY

TYPICAL RESULT

UNITS

Tensile strength

75

MPa

Impact strength, notched, Charpy

20.0

kJ/m2

Impact strength, notched, Izod

10

ft.lbf

Bond strength, 10 mm ball

1000

lbf

Hardness  Rockwell M scale

98

-

Water Absorption

 

 

3mm thick.

17

mg

6mm thick.

19

mg

Water absorption, long term saturation

1.6

%

Insulation resistance after immersion in water

1x1010

ohms

Electric strength, flatwise in oil at 90o C

 

 

3mm thick.

10

MV/m

Electric strength, edgewise in oil at 90oC

65

kV

Relative density

1.3

-

Maximum working temperature**

 

 

continuous

90

0C

intermittent

120

0C

 

 

 

Thermal classification

Class E

-

 

 

 

Thermal expansion in plane of laminations

4.5

x10-5/K

 

Test methods as BS EN 60893-2, where applicable.

 

**Users of highly stressed components at temperatures approaching the maximum are recommended to seek further advice from TUFNOL Composites Ltd.

 Note. Under certain conditions, water with MoS2 can give rise to acidic conditions.  If in doubt, please contact Tufnol Composites Ltd

 

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A full machining service is available from Tufnol for this and many other engineering plastics and composites.

Tufnol Composites Ltd, Wellhead Lane, Perry Barr, Birmingham B42 2TB, United Kingdom.

Quality Registration BS EN ISO 9001:2000, BS EN 9100:2003 and AS 9100 Reg. No. FM 90942

The information given here is believed to be correct, but completeness and accuracy are not guaranteed. The user shall be fully responsible for determining the suitability of products for the intended use.

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