GB2114633A - Rail pad - Google Patents

Abstract

A rail pad 6, which is located between a rail and a supporting member, is formed from a cross-linked copolymer of ethylene and an unsaturated ester which contains at least 10% by weight of the ester and preferably not more than 30% of the ester. The unsaturated ester is typically vinyl acetate. The cross-linked copolymer has a gel fraction of at least 75% and preferably greater than 80% <IMAGE>

Description

SPECIFICATION Rail pad The present invention relates to pads which are inserted between a rail and a supporting member in a railway track.

For a number of years, pads have been inserted between a rail and a supporting member in a railway track. These pads are located under the rail and rest on the supporting member, particularly when this is a reinforced concrete sleeper. The pads provide electrical insulation between the rail and the supporting member, suppress vibration, and absorb impact loading.

The pads can be formed from a variety of materials which include various rubbers, polyethylene (both low and high density) and copolymers of ethylene with other monomers, particularly unsaturated esters such as vinyl acetate. Improved rail pads have been proposed, see for example UK Patent Application No.

GB 2 051 1 87A, which are composites of two or more materials, such as soft and hard rubber.

However, such composite rail pads are more difficult to produce and it is desirable that the rail pad can be mass-produced using comparatively simple techniques.

According to the present invention there is provided an assembly comprising a rail, a supporting member located beneath the rail, a pad located between the rail and the supporting member and means effective to secure the rail to the supporting member wherein the pad is formed from a cross-linked copolymer of ethylene and at least one unsaturated acid or ester, which copolymer has a gel fraction (as hereafter defined) of at least 75% and an acid or ester content of at least 10% by weight.

The assembly may form part of the track for a vehicle which is capable of running on rails, for example a crane, or a car in a tramway system, and is particularly suitable for use in a railway system.

It is preferred that the copolymer contains an unsaturated ester which may be an ester of an unsaturated carboxylic acid such as the alkyl esters of acrylic and methacrylic acids and other unsaturated acids containing the group CH2=CH-. Preferred esters are the unsaturated esters of the saturated carboxylic acids such as the vinyl esters of acetic propanoic and butanoic acids. A particularly preferred unsaturated ester is vinyl acetate.

The acid or ester content of the copolymer is at least 10% by weight and typically does not exceed 50% by weight and, with an un-filled copolymer, preferably does not exceed 30% by weight.

Preferred copolymers are those which contain at least 12% by weight of the acid or ester.

The cross-linked copolymer preferably has a gel fraction of greater than 80%. In general the extent of cross-linking is such that the gel fraction of the copolymer does not exceed 95%. By "gel fraction" is meant the proportion of unextracted polymer (making due allowance for any filler present) as determined by the procedure of ASTM D 2765-68 (Method A).

Cross linking ofthe copolymer may be achieved using any of the known techniques such as peroxide, radiation or silane cross-linking. Silane cross-linking can be effected by reacting the copolymer with a silane, shaping the modified copolymer to form the pad and thereafter treating the pad with a suitable reagent, such as water, to effect cross-linking. Radiation cross-linking is effected by shaping the copolymer to form the pad and thereafter subjecting the pad to irradiation, for example electron beam irradiation or gamma radiation from a cobalt-60 source, to effect crosslinking. Using either of the foregoing techniques, the copolymer can be shaped to form the pad using conventional injection moulding conditions.

Cross-linking is preferably effected by using a pre-mix of the copolymer and a peroxide, injecting this mixture at a moderate temperature, for example up to 1 500C and typically in the range 1 00C up to 1 400C, and heating in the mould to a temperature which is sufficiently high to cause cross-linking. The temperature in the mould is typically in the range 1 400C up to 2200 C, preferably 1 500C up to 2000C, and especially 1 600C up to 1 900C, but will be dependent on the half-life characteristics of the peroxide used.

Suitable peroxides include dicumyl peroxide; 1,3bis(tertiary-butylperoxy isopropyl) benzene; 2,5dimethyl-2,5-di(tertiary-butylperoxy) hexane and 2,5-dimethyl-2,5-di(tertiary-butylperoxy) hexyne3. The proportion of peroxide used to effect crosslinking is conveniently in the range 0.2 to 6%, preferably 0.5 to 4% and especially 0.5 up to 1.5%, by weight relative to the weight of the copolymer.

The pre-mixture of the copolymer and peroxide can be obtained using the peroxide in any suitable form, for example as the undiluted peroxide, the peroxide on an essentially inert support material such as chalk or preferably as a masterbatch of the peroxide and a polymeric material, particularly the copolymer, the masterbatch being in granular or powder form.

The supporting member may be any suitable member of the type known for supporting a rail in a railway system and the means to secure the rail to the supporting member may be any known securing means.

The pad is preferably shaped to fit within the securing means to give precise location of the pad.

Typically, the pad is provided, at opposite ends, with ribs which fit over the edges of the supporting member or opposite sides of the pad are provided with integral extension pieces which fit on either side of a part of the securing means.

The invention will be further described by reference to the accompanying drawings wherein: Figure 1 is a plan view of one form of pad; Figure 2 is a plan view of an alternative form of pad; and Figure 3 is a cross section elevation through an assembly of a rail, part of a supporting member, pad and securing means.

In Figure -1, a rectangular pad 1 is provided with a rib 2 at each end, each of the rib 2 extending-the length-of the shorter side of pad 1.

In Figure 2, a pad 3 is provided with integral extension pieces4 which are located at either end of each long side of pad 3.

In Figure 3, a rail 5 rests on a pad 6 which in turn rests on a-reinforced concretesleeper 7. The pad 6, which may he of the type shown-in- Figure 1 or in Figure 2, is-located between two clip mountings 8. The clip mounting 8 are secured to the sleeper 7 by-a tongue 9 which extendsinto the body of the sleeper 7.Each clip mountin-g8-is provided with a hole 10 passing-through--it. The rail 5 is secured to the sleeper 7 by two clips 11 formed of spring-steel rod. One end of each clip 11 is located the hole 1 O-in clip mounting 8 and the other end of the clip 1 1 presses onto.an insulator pad 12 which, in its turn; presses-onto a flange 13 formed at the footof the rail 5. The clips 11 are mounted in-opposing directions and-this minimises longitudinal movement of the rail.

The pads 1, 3 and 6 areformed from a crosslinked ethylene-vinyl acetate copolymer containing at least 10% by weight of vinyl- acetate.

The insulator pads 12 is formed from-a plastics material, for example nylon 66 or glass-fibre- reinforced nylon 66.

Using pads formed from a-cross-linked copolymer in accordance with the present invention gives-assembly which is suitable for use in a hotter climate than is possible. using a-non- cross-linked copolymer. Furthermore, in all climates,.the.use.af-a cross-linked copolymer permits the use of a softer copolymer, that is a copolymer having a higher acid or ester content, and this gives improved attenuation of vibration (and therefore noise) and, since the softer copolymer has a higher coefficient of friction, provides a greater resistance to longitudinal movement of the rail. The pads may be provided with ribs and/or grooves in the known manner.

It will be appreciated that there are many variations possible and these variations may be effected without departing from the invention as defined herein.

Claims (16)

1. An assembly comprising a rail, a supporting member located beneath the rail, a pad located between the rail and the supporting -member and means effective to secure:the rail to-the supporting member wherein-the padtisformed from a cross-linked copolymer of ethylene and:at least one unsaturated acid or ester7 which copolymer has a gel fraction of at least 75% and an acid or ester content-ofatleast 10% by weight.

2. An assembly as claimed:in claim 1 which forms part of a track for a vehicle which is capable of running on rails.

3. An assembly as claimed-in claim 1 or claim 2 wherein the- copolymer is a copolymer of ethylene and an unsaturated ester.

4.-An assembly claimed in claim 3 wherein the unsaturated ester-is-an ester of an unsaturated carboxylic acid.

5. An assembly as claimed in- claim 4 wherein the unsaturated ester is-an alkyl ester of acrylic acid or rrfethacrylic acid.

6. An assembly as claimed in claim 3 wherein the ester is the unsaturated ester of a saturated carboxylic acid.

7.-An assembly as claimed in claim 6 wherein the ester is vinyl ester of acetic acid, propanoic acid or butanoic acid.

8. An-assembly as claimed in-claim 7 wherein the copolymeris-a copolymer of ethylene and vinyl acetate.

9. An assembly-as claimed-in any one of claims 1 to 8 wherein the copolymer contains not more than 50% by weight of the unsaturated acid or ester.

10. An assembly as claimed in claim 9 wherein an-unfilled copolymeris used which contains not more than 30% by weight of the unsaturated acid or ester.

11. An assembly as claimed in any one of claims 1 to 10 wherein the copolymer contains at least 12% by weight of the unsaturated acid or ester.

12. An assembly as claimed in any one of claims 1 to 11 wherein the-copolymer has a gel fraction of greaterth-an 80%.

13. An assembly as claimed in any one of claims 1 to 12 wherein the copolymer has a gel fraction not exceeding 95%.

14. An assembly as claimed in any one of claims 1 to 1 3 wherein-the pad has been formed by injecting a-mixture of a non-cross-linked copolymer of ethylene and at least 10% by weight of at least one unsaturated acid or ester, with a peroxide into a-mould at a temperature of at least 1500'C and maintaining the mixture in-the mould for a sufficient time for a desired degree of crosslinking to occur.

15. An.assembly as claimed in any one of claims 1 to 14 wherein the supporting member is a reinforceci concrete sleeper.

16. A length of railwaytrack which is formed of assemblies each of which is as claimed in any one of claims 1 to 15,