Developments in the Use of Natural Rubber As an Engineering Material

J. Rubb. Res. last. Malaya, 22(2), 176-190 (1969) Developments in the Use of Natural Rubber as an Engineering Material

P. B. LINDLEY and A. R. PAYNE Natural Rubber Producers' Research Association, Welwyn Garden City, Herts., U.K.

The use of natural rubber in engineering applications has increased substantially during recent years. A number of the latest developments in mechanical and civil engineering are described together with some of the considerations necessary in the design of rubber-metal assemblies. Current developments in the type of vulcanisate suitable for use in natural rubber springs are also considered.

VEHICLE SUSPENSIONS BMC have also fitted Hydrolastic suspension The important requirements of rubber for use to their 1968 Austin 3-litre luxury car. This is in vehicle suspension springs are (a) an ability the first time the system has been applied either to withstand repeated stressing without failure, to a rear drive car or to a car of this power. The i.e., a high fatigue resistance, and(b)low creep, suspension layout (Figure 3) consists of stan- as settlement of a vehicle with time is undesi- dard 6^ in. units at the front but at the rear the rable. The excellent strength and creep proper- dampers and springs have been separated. The ties of natural rubber make it the preferred rubber springs are still deformed by fluid pres- elastomer for vehicle suspension springs. sure, but as the fluid pressure can be readily transmitted through feed pipes there is no Hydrolastic Suspension longer any necessity for the springs to be Since 1962 the British Motor Corporation mounted in the conventional place. As a result have fitted to their 1100, Mini and 1800 ranges the duplex rubber springs have been fixed under of passenger car the revolutionary system of the body away from the rear axle. Another fea- hydrolastic suspension designed by Moulton ture of the car is that the rear is maintained at Developments Ltd., and manufactured by the a constant height by a hydraulic automatic Dunlop Co. Ltd. levelling device. The hydrolastic system incorporates both Electric Cars the springing, with natural rubber as the spring medium, and the damping, by the restriction As city-centre traffic conditions progressively worsen the benefits to be gained from using of the flow of constant-viscosity alcohol/water electric cars will increase. These cars will be liquid through valves with natural rubber flaps. cheap, manoeuvrable, easily parked and the Because the front and rear springs are inter- absence of exhaust gases will not add to the connected hydraulically, a flatter ride with health hazard of air pollution. little roll on cornering is achieved. Should any Ford of Britain have already produced a creep occur in the rubber it can be corrected by prototype electric-powered car, the Comuta adding more liquid to the system. (Figure 4). By the use of independent Neidhart The hydrolastic system of the 1800 is shown natural rubber suspension on all four wheels schematically in Figure 1 with a plan of the its overall length has been kept to 6 ft 8 in.; the front 6| in. unit in Figure 2. The design would arrangement is shown in Figure 5. not have been possible without the reliable Each Neidhart unit comprises a small square- bond between the rubber and metal. sectioned tube fitting diagonally inside a

COMMUNICATION 511

176 Rear Hydrolastic /•- Gin. units

T Is in. bore interconnecting pipes Front Hydrolastic G-j in. units

811 Ibf

Figure 1. Lay-out of the Hydrolastic suspension system in the BMC 1800 showing the low-pitch frequency and high-roll stiffness which give rise to the "haracteristic flat ride obtained on Hydrolastic suspended cars.

Interconnecting pipe

Schroder valve for charging ARMSTRONG LEVELLING RAM

Rubber hose

Rubber hydraulic spring

Port plate Damper assembly Bleed hole

Tapered skirt - Displacer assembly Diaphragm and liner Tapered piston

Figure 2. The 6%-in. Hydrolastic unit which is fitted to the front of the BMC 1800 and 3-litre Figure 3, Hydrolastic suspension system fitted cars. to Austin 3-litre car (viewed from underside).

larger square-sectioned tube, which is part of the suspension arm, with solid rubber rods between the flats of the inner tube and the corners of the outer fsee also Figure 7). When the arm rotates relative to the inner tube the rubber rods are compressed, thus providing the springing. The load-deformation characteristics of Neid- hart springs can be modified by changing the shape of the 'flats' on the inner tube or the corners of the outer tube. In the Comuta the front and rear suspension units are of the same section and employ a natural rubber of 60 IRHD. The chassis consists of a central beam fitted front and rear with two steel plate frames. The front part supports the front suspension units and the four 12-volt batteries. Two 24-volt electric motors are fitted at the rear and drive the wheels by articulated shafts passing through the lever-arm of the Neidhart units. The outer joints of the drive shafts are Metalastik Rota- Figure 4. Prototype electric-powered Comuta flex natural rubber couplings (Figure 5) which car made by Ford of Britain. absorb vibrations and provide a direct drive to the rear wheels.

Neidhart springs

Rotaflex coupling Figure 5. Rear view of the Comuta car showing the position of the Neidhart and Rotaflex units.

178 P. B. LINDLEY ANT) A. R. PAYNE: Developments in the Use of NR as Engineering Material

Figure 6. Brigel road-making vibro-ramming machine.

Road-making Machinery Brigel employing a total of 72 Neidhart units. Another use for Neidhart springs is in the Two more of these units are used to give a new Brigel road-making vibro-ramming ma- flexible connection with the propelling vehicle. chine (Figure 6) built in Switzerland. A ramming force of up to 4 tons is provided by an out- Hovercraft Landing Pads of-balance motor which vibrates up and down The first production model of the new 165- at 2800 c/min. A ramming plate is rigidly at- ton SRN4 hovercraft by the British Hover- tached to the motor and the whole is flexibly craft Corporation is shown in Figure 8. The suspended by linkages with rubber bearings new craft, four times larger than any previous (F/gure7).The bearings must resist the scuffing operational hovercraft, touches down on five and abrasive action of sand, dust and flying landing pads of natural rubber. stones, work in all directions (e.g., allow for These pads, produced by the Avon Rubber tilting when the plate hits a stone) and require Company, are large natural rubber rings, 27 in. no maintenance. Such requirements precluded diameter, 10 in. deep and weighing 195 Ib, bond- the use of metal bearings. ed at the top and bottom to circular aluminium The flexibility is provided by Neidhart natu- plates. The top plate is bolted to the underside ral rubber beari ngs arranged in four sets of three of the hovercraft and the lower one to a metal per ramming plate. There are six plates in the skid plate. Each pad is capable of supporting 80

179 Journal of the Rubber Research Institute of Malaya, Volume 22, Part 2, 1969

air space containing the two axial compression bearers. Horizontal movement (Figure 10, right) is accommodated by shearing of the compression bearers and tilting of the conical units. This lateral movement enables the carriage bogie to pivot, even when the Air-Metacone is mounted on outside bogie frames. Figure 11 shows such an installation on a motor bogie, the suspension of which also incorporates rubber chevron axlebox springs and (in the centre of the drawing) rubber traction mounting manufactured by Metalastik Ltd. MOUNTING OF LARGE STRUCTURES The use of laminated natural rubber/metal bearings for supporting bridges came into pro- minence in 1956 when they were used in the Figure 7. Neidhart springs, with the natural Pelham Bridge at Lincoln. Since then the moun- rubber rods shown in black, in the vibro-rammer. ting of bridges on rubber bearings has become widely accepted practice in Britain and many ton in compression (i.e., half the weight of the parts of the world. hovercraft) and up to 40 ton in shear (as shown Some of the design features of rubber bridge in Figure 9) to absorb the impact of heavy land- bearings have recently been extended to anti- ings. vibration mountings for buildings and supports Considerable care is taken during manufac- for other large structures. ture to ensure complete and uniform vulcani- The majority of these applications require sation throughout the rubber and a good bond bulky components which are immune in their between the rubber and aluminium. bulk from the effects of any surface attack by, for example, ozone and oil. The low creep and Rail Coach Suspension its ease of manufacture make natural rubber The Air-Metacone suspension developed by the most widely used elastomer for these types Metalastik Ltd. for railway coaching stock is of components. now undergoing experimental trials on Stock- holm and London Underground and on the Haringvliet Floodgates Swedish State Railways. The construction of The new 2|-kilometre wide floodgate struc- the unit (Figure 10) is such that the tare weight ture across the Haringvliet estuary in Holland of the carriage is carried on rubber. The addi- incorporates seventeen sluices separated by con- tional weight of passengers actuates valves crete piers. A pre-stressed concrete bridge which introduce air under pressure into the mounted on the piers carries a double carriage- springs and return the carriage to its original way across the estuary. Thermal movements of height (as in normal air suspension). Air flow the bridge are accommodated by laminated between the spring and an extra-volume tank rubber bearings, each of which supports a is restricted and this provides all the damping structure-plus-traffic load of about 650 tons. that is necessary. A 16 in. diameter Air-Meta- There are sluice gates at both the upstream cone spring can support 4 tons on the rubber and seaward sides. Both sets of gates are con- and 2| tons on air pressure. However, if the air nected by radius/thrust arms to the bridge to pressure should fail the rubber alone will sup- which water loads on the gates are transmitted. port the loaded vehicle. Variations in the net horizontal load on the The spring consists of two conical units, bridge arise from the ebb and flow of the tide which take the vertical laod, surrounding the and the different water levels on either side of

180 P. B. LJNDLEY AND A. R. PAYNE: Developments in the Use of NR as Engineering Material

Figure 8. SRN4 hovercraft at its unveiling ceremony in October 1967. the sluices. This net horizontal load is trans- most people, the building has been mounted on mitted to the piers partially through shear of rubber springs to give a vertical natural fre- the laminated bearings and partially through quency of 7 c/s. Although this has meant some compression of vertically disposed rubber magnification of the vibrations at low frequen- blocks (Figure 12). cies the overall attenuation of the vibrations by Albany Court Flats the rubber has achieved the desired purpose (Figure 13). The first complete building in Britain to be Laminated rubber springs provided the most isolated from low-frequency groundborne vib- economical solution, the actual cost of the rations is the Albany Court block of residen- springs amounting to about 2% of the total tial flats situated over St. James's Park Under- cost of the building. The locations of the natu- ground Station in London. The predominant ral rubber springs supporting the building are frequency of the vibrations from the under- shown in Figure 14. ground is in the range 20-25 c/s but vibrations The springs, supplied by the Andre Rubber are also present at lower frequencies. To reduce Company, are special versions of their normal vibration to a level which is imperceptible to bridge bearings with metal plates completely

181 Journal of the Rubber Research Institute of Malaya, Volume 22, Part 2, 1969

embedded in the rubber. This provides pro- tection for the metal and for the bond between the rubber and the steel. The total load carried is about 1400 tons and the largest spring, which measures 24" X 20" X lliV, supports a load of 216 tons. The design of the springs took into account the ratio of dynamic to static stiffness, internal damping and the effects of non-linearity in compression. To obtain the natural frequency of 7 c/s a total static deflection of f in. was required. There are three important factors relevant to the use of natural rubber springs for the Albany Court flats and similar applications, (i) Atmos- pheric attack on the rubber is insignificant with the large masses of rubber involved in springs of this size. The bearings of the Pelham Bridge, for example, show no sign of ozone cracking after ten years in service, (h) Settlement of the bearings in the building has been monitored Figure 9. A landing pad for the SRN4 hover- since it was erected and, up to the present time, craft under a shear load of 40 tons during testing the amount of creep has been found to be negli- at the Avon Rubber Company factory. gible (DERHAM et ai, 1969). However, although a rubber spring with an initial deflection of f in. might creep i in. in fifty years, the

Figure 10. Cross-section of the Metalastik Air-Metacone suspension unit. The right hand drawing illustrates how horizontal movement is accommodated.

182 P. B. LINDLEY AND A. R. PAYNE: Developments in the Use of NR as Engineering Material

Figure 11. Air-Metacone spring fitted to an electric multiple-unit motor bogie which also incorporates Metalastik rubber chevron primary springs and rubber-metal traction mounting.

differential creep between one bearing and dential environment for 10 000 people. The its neighbour would only be a small propor- lay-out, of a barrier of flats and low level two- tion of this and insignificant compared to storey buildings surrounding a lower central the differential settlement of the foundations, area, is designed to mask traffic noise from the (iii) Fire risk is overcome because the laminated outside roads and create a quiet, noise-free area rubber bearing would continue to carry load in the centre of the scheme. However, as four far longer than the one or two hours demanded busy tracks of the London Underground cross by Fire Rating requirements, due to the slow the area it was necessary to keep to a fairly low speed at which the laminated rubber would level the noise and vibration disturbance from absorb heat and burn. In any event it is rela- this source too. tively simple to ensure that the building cannot To achieve this, the tracks are carried on a settle more than, say, | in. by providing solid steps as part of the foundation. fairly heavy slab bridge deck supported at in- tervals on rubber mountings (Figure 15). The The Barbican Scheme deck is resonant at 6 c/s under the dead weight The Barbican re-development area in London of the track, ballast and deck so giving isolation will, when complete, provide a complete resi- for vibrations of about 15 c/s upwards.

183 Journal of the Rubber Research Institute of Malaya, Volume 22, Part 2, 1969

Figure 12. Pre-stressed concrete bridge in the Haringvliet floodgates shown as the inverted triangular section supported on the natural rubber bearings.

The mounted deck differs from a normal The bearings, manufactured by the Leyland bridge deck in that it is free to move at all its & Birmingham Rubber Company, consist of support points instead of having a succession blocks of relatively soft natural rubber having of free and fixed bearings as is customary low creep and dynamic/static stiffness ratio where expansion is the prime factor to be con- bonded to outer plates. The plates are also per- sidered. Provision is made for access to the forated, as this gave a mechanical connection supports for inspection and possible subsequent at negligible extra cost. renewal of bearings. Results so far indicate that a very high degree Rubber side bearers provide lateral restraint of attenuation is being attained. without offering a path for by-pass transmission. They are compressed so that they are always in DOCKSIDE FENDERS contact with both sides of the deck, i.e., no free If the berthing speed of a heavy ship is too high, sideways movement can occur before the res- damage may occur either to the dock and har- training forces are brought into play. bour installations or to the ship. To prevent

184 P. B. LINDLEY AND A. R. PAYNE: Developments in the Use of NR as Engineering Material

'sandwiches' of rubber bonded between steel 04 to Ckarl y Perceptible plates to form a V-type assembly. DEVELOPMENTS OF COMPOUNDS O 3 Previous Building FOR NATURAL RUBBER SPRINGS on site The requirements of rubber vulcanisates for use

O-2 in engineering applications such as vehicle

Booking Hal) /'Ground suspensions, anti-vibration mountings, bearings, seals and fender units are very much the Threshold same as are wanted in any other material which oT Perception could be considered for these spring-like appli-

O-l cations. The principal requirement of a spring New Flats material is that it should be elastic, as any non- elasticity tends to introduce a variety of un- -5 wanted effects. Thus, for rubber, minimum {Vibrations Vertical 5-3Oc/s) creep and stress relaxation and a low ratio of dynamic stiffness to static stiffness are of prime Figure 13. The previous building on the site of importance. the present Albany Fiats was subjected to clearly Natural rubber gum vulcanisates can be perceptible vibrations from the Underground highly elastic and, by virtue of strain-induced Railway. Vibration in the new rubber-mounted crystallisation, extremely strong and fatigue- flats is below the threshold of perception. resistant too. The inclusion in natural rubber this, many harbour authorities install fender units which make use of the high energy storage capacity of rubber. Two essential requirements of fenders are that they should operate effective- ly under direct impact and also under glancing impact when a ship comes into berth at an angle. The fender units produced by the Lord Manufacturing Co. buckle in an upward direc- tion under pressure as shown in Figure 16. This upward buckling provides a high build-up of load for a relatively small deflection and provides efficient cushioning irrespective of the ship's angle of approach. Natural rubber is used because of its excellent physical properties, ease of fabrication, and good behaviour at low natural temperatures. The use of this type offender is rubber steadily increasing. springs Other widely used types of rubber fender unit include (i) tubular fenders consisting of hollow tubular or rectangular sections secured to the face of the quay or dockside by wire or nylon ropes or steel chains, (ii) pneumatic fenders comprising several giant pneumatic tyres Figure 14. Plan and elevation of the Albany affixed to the side of piers and dockyards, and Court flats showing the position and loadings of (iii} the Raykin fender consisting of a series of the rubber springs.

185 Journal of the Rubber Research Institute of Malaya, Volume 22, Part 2, 1969

/•Walkway level

Void Void

4lt-6in iriin. 35fl-0in.

Track level Infill filling . -. 'i *,;•-, l.imeolcre Ifcck base •

PS I beams -Rubber R C. crossbeam bear ngs

Longitudinal iectkm

Walk wo y

Underground Lofce car park 9 id R.C roof

IGinRCwoll

18 In. track base P/S I beams (insilu Infilling) bearings RC.r«t crow boon Rubber bearings R.C. trough cms

Cylinder piles (3ft -4 io 0)

Typical cross section Figure 15. Underground railway track in the Barbican Scheme showing the location of the main support bearings and the side bearings. vulcanisates of carbon black increases the hard- vulcanisate behaviour for different properties ness range well beyond that of gum vulcani- and in every case the gum vulcanisate is super- sates and makes the compound cheaper and ior (i.e., more independent of the variable). easier to mould. The black, however, is a non- Thus for engineering applications, there are elastic inclusion which has a number of undesi- major advantages to be gained by using unfilled rable properties associated with it. Figures 17 to or, at the most, only lightly filled vulcanisates of 20 compare filled NR vulcanisate and gum NR natural rubber.

186 P. B. LINDLEY AND A. R. PAYNn: Developments in the Use of NR as Engineering Material

3O-i

O O Strain Figure 17. Hysteresis loops in black-fillep rubber (full line) and unfilled rubber (broken line.) The energy loss during each cycle is converted into heat and, in black-filled rubbers, can lead to large undesirable temperature rises.

based on rubbers of about 55 IRHD, so these errors could be corrected by changing the hard- Figure 16. The energy-absorbing buck Hug ness. Hardness can be increased to 75 IRHD or action of the Lord Fender unit. decreased to 35 IRHD, corresponding to a stiffness change of a factor of 3 on each side of the mean. It would also be desirable if the number of Rubber has an advantage over other mate- compounds used was limited to avoid expen- rials in that its bulk compression modulus is very sively obtained data on design, manufacture high compared with its Young's modulus and and performance from being spread thinly over that, by suitable design, an effective modulus many different compounds. With comprehen- anywhere between them can be obtained. Thus, sive data on one compound theengineer would design (of shape and size) with one compound be able to design with accuracy. In the past, can be used in preference to changing the hard- because of the lack of precise data, preliminary ness (and hence the characteristics of the vul- designs have often been rough approximations canisate) to achieve the desired spring stiffness.

187 Journal of the Rubber Research Institute of Malaya, Volume 22, Part 2, 1969

3 5- 25-

2-O- 3O- — ut E

2 5- Q

•O- 2O- O 25 SO 75 IOO Parts Per Hundred Rubber Of HAF Black In Natural Rubber 5- Figure 19. The dynamic stiffness of black- filled rubber components can be very much higher

"O than their static stiffness. In unfilled vulcanisates o there is very tittle difference between the static I O- and dynamic behaviour (Results from KNUREK AND SALISBURY, 1964). 3O degradation, such as ozone cracking or swell- NR Gum ing, is of minor consideration compared with the maintenance of the design stiffness charac- O teristics. For example, some small blocks IO" IO" IO I0a o. 4" x 4" X 1" of the above formulation Strain Amplitude were installed outdoor under a large in- flexible pipeline. In their ten years of service, Figure 18. Variation of dynamic shear modulus these blocks show very little creep, and only with strain amplitude for different loadings of slight ozone cracking on the surface even ISAF black (parts per hundred rubber).

One vulcanisate for which useful data already exist is (parts by weight): 30 creep Natural rubber 100 Zinc oxide 5 Stearic acid 2 CBS 0.6 Sulphur 2.5 10 PEN 1 Vulcanisation time 40minatI40°C 1 10 100 1000 10000 100000 i Slight variations on this formulation can time minutes i give a vulcanisate with necessarily the same i Iday elastic stiffness but more appropriate for even 1 week vulcanisation of thicker units, or with improved Figure 20. Typical creep curves in shear. (O weathering resistance. Unfilled, • 50 p.p.h.r.HAF black, + 50 p.p.h.r. As most engineering components of natural MTblack). The inclusion of carbon black filler rubber are fairly bulky, any possible surface increases creep.

188 P. B. LINDLEY AND A. R. PAYNE: Developments in the Use of NR as Engineering Material though no special protective agents were in- Corporation, British Motor Corporation, Dun- corporated in the rubber. If the degradation lop Co. Ltd., Ford Motor Co. Ltd., Leyland & continues at the same rate, these unprotected Birmingham Rubber Co. Ltd., (now the Poly- blocks will last for hundreds of years. mer Engineering Division of Dunlop Co. Ltd.), Another example is the 100-year-old natural Lord Manufacturing Co., Metalastik Ltd., rubber sealing ring from a sewer on the eastern Moulton Developments Ltd., Neidhart S.A., outskirts of London excavated in 1963. Surface Ove Arup & Partners (Barbican Scheme). The degradation had occurred to a depth of I" -i". authors also wish to thank Mr. R.E. Whit- but the ring was still functioning satisfactorily. taker for his help in assembling information The inner rubber was analysed and tested, and for this paper. This work forms part of the found to show very little loss of strength com- programme of research of the Natural Rubber pared with rubber freshly made to the same Producers' Research Association. formulation. In these and many other cases the effect of REFERENCES possible surface degradation on the service be- DERHAM, CJ. LAKE, G.J. AND THOMAS, A.G. (1969) haviour has been negligible. Many of the newer Some factors affecting the service life of natural rubber articles. J. Ruhb. Res. Inst. Malaya, 22(2), applications, some of which have been des- 191. cribed in this paper, incorporate protective KNUREK, T.A. AND SALISBURY, R.P. (1964) Car- agents which prevent even this surface attack. bon black effect on engine mount compounds. Top quality natural rubber vulcanisates have Rubb. Wld, N. ¥., 150(5), 46. invariably been used to give the necessary pro- BIBLIOGRAPHY perties and, for this reason too, the amount of filler has been kept to the minimum. ALLEN, P.W.,LINDLEY, P.B. AND PAYNE, A.R. ed. (1967) Use of Rubber in Engineering. London: Mac- ACKNOWLEDGEMENT laren & Sons Ltd. DAVEY, A.B. AND PAYNB, A.R. (1965) Rubber in The authors wish to acknowledge the following Engineering Practice. London: Maclaren & Sons Ltd. manufacturers and organisations for supplying LINDLEY, P.B. (1966) Engineering design with natural details and for permission to reproduce some rubber. N.R.P.R.A. Nat. Rubb. Tech. Bull. No. 8. of the figures in this paper: Robert Aebi NAUNTON, W.J.S. ed. (1961) The Applied Science of A.G., Zurich (Brigel), Andre Rubber Co. Ltd., Rubber. London: Edward Arnold Ltd. PAYNE, A.R. AND SCOTT, J.R. (1961) Engineering W.S. Atkms & Partners (Albany Court Flats), Design with Rubber. London: Maclaren & Sons Avon Rubber Co. Ltd., British Hovercraft Ltd.

DISCUSSION Chairman: Dr. H. W. Greensmith (The paper was presented by Mr. A. G. Thomas) Dr.P.M. Sorgo enquired if improvements in engineering properties might be obtained by blending natural rubber with other elastomers. Mr. Thomas said certain strength properties might be improved by blending, but the full potential of unblended natural rubber had not yet been realised in most designs or compounds. Dr.J.L Cunneen added that the importance of creep in the use of rubber as an engineering material had been demon- strated by the authors. Vulcanisate structure affected creep, which could be reduced in natural rubber by vulcanisation with peroxides instead of with conventional systems accelerated by sulphur. The Chairman asked whether engineering components would become more bulky if gum vulcanisates replaced the conventionally filled rubbers. Mr. Thomas replied this was not necessarily so. Dr. B. Saville asked how natural rubber—a resilient material—could be used in the support of buildings to absorb energy from underground sources. Mr. Thomas said that rubber isolated the vibrations, but did not it- self absorb the energy. Mr. J. O'Connell asked if there was any risk of a building mounted on rubber bearings

189 Journal of the Rubber Research Institute of Malaya, Volume 22, Part 2, 1969 beng destroyed by resonance, due to the wind, for example. Mr. Thomas said the effect of the wind was taken into account in the design of the structures so that there was no risk. Mr. S.T. Semegen asked about the present status of rubber in roads and in flexible gypsum plaster. Mr. Thomas said the use of rubber in roads was increasing in view of its well established technical merits, des- pite difficulties due to the costing policy of some engineers. Mr. A.D.T. Gorton added that rubber had a pro- mising future in flexible gypsum plaster as a sealant and caulk; especially in mining, such use had reached the commercial stage.

190