GB2176454A - Plastics wear strip for conveyor chains - Google Patents

Abstract

A wear strip (1) for chain conveyors with plastic chains (2) is made from a surface hard plastic material e.g. polyester which can be made even more surface hard by intermixing of glass fibres and be produced by injection moulding. It is thus avoided that dust and foreign particles become embedded in the surface of the wear strip and thus cause the plastic chain to wear quickly. If so desired it is possible in the material to furthermore incorporate wax whereby the friction with the plastic chain and the foreign particles, respectively, may be reduced. The wear strip may be made of units which link together in the manner of a chain. <IMAGE>

Description

SPECIFICATION Plastic wear strip for supporting conveyor chains, particularly chains with plastic chain links The present invention relates to a wear strip for chain conveyors, particularly for sliding support of conveyor chains with plastic chain links. In such conveyors it is necessary that the conveyor chain slides on special wear strips which ideally are partly hard-wearing and partly act non-wearing on the chain and partly allow the loaded chain to be conveyed with low friction just as the wear strips should of course be form stabile and reasonably low-priced. Chain conveyors are used in industry to a very large extent and great resources have been spent on the development of suitable wear strips without hitherto producing quite satisfactory results, however.

Development has up to now resulted in a predominant use of wear strips of either stainless steel, polyethylene or Nylon.

When using stainless steel it is necessary for low friction operation to lubricate the wear or slide strips for example with water which is in itself inconvenient. It is moreover found that after some time the steel material gives off black particles which with the water runs down the conveyor frame onto the floor which is unacceptable particularly in the food processing industry.

Wear strips made of plastic do not require lubrication which means that they will find use partly generally and partly in particular where the goods are to be handled in dry surroundings. In such situations polyethylene is the preferred material. This material is in its way excellent if it is an environment wherein there are not dust or particles of impurities but such conditions are not present by the majority of conveyor installations and it is common experience that plastic chains wear quite quickly in most conveyors. It is true that the slide strips are highly hard-wearing but the overall result is of course poor if they give rise to a marked wear on the conveyor chains.The explanation seems to be that the polyethylene strips show such surface softness that dust and larger particles can be pressed down into same to retained positions wherein the particles act in the manner of sandpaper to the travelling chain.

As mentioned above also Nylon has been found applicable for meeting the stated ideal requirements to the wear strip material but this view seems to be based on laboratory experience and not on practical conditions. Also Nylon is in practice adapted in its surface to receive and retain particles which will then act highly wearing on the plastic chain just as precisely Nylon is moist absorbing and thus is extensible in the longitudinal direction of an elongate strip in such a manner that this will in itself give rise to important mechanical stability problems in practice.

Polyethylene as well as Nylon act as "surface soft" materials and this has hitherto been found important for obtaining low friction in connection with conveyor chains of plastic, normally consisting of an acetal material, but it is now shown here that the existing softness accounts for a highly undesirable part result, namely that the wear strip after a short time of practical use is converted into a regular grinding or sanding element for the travelling conveyor chain.

The invention is directly based on this observation and prescribes that for the wear strip there is used a surface hard material of plastic particularly polyester. The fundamental idea of the invention is thus in principle that there is used a harder material for the wear strips than for the chain links sliding thereon. It turns out that such a surface hard plastic material may provide a desired low friction by co-operation with the ordinarily used plastic chains and moreover show the important property that foreign particles will not or merely under extreme conditions be pressed into and be retained in the surface of the strip material so that the mentioned grinding or sanding effect may be avoided in practice.

The hitherto used slide strips have in practice been produced by sawing through pre-moulded plastic blocks which has unavoidably brought about saw cuts and thus the following irregularities on the surface of the slide strips. Such irregularities are in a way not essential to the general friction between the conveyor chain and the slide strip but they are essential to the total friction in that they will promote the possibility of retention and accumulation of impurities which will easily collect in the bottom areas of the irregularities in the strip surface.

It is in the light of this a special feature by the invention that these irregularities in the material surface are countered in the manner that the slide strips appear as composed of injection moulded strip units whereby the strip sections may be produced with a very high degree of surface smoothness so that the surface will not promote local collection of present impurities.

A further quite important improvement according to the invention may be obtained by intermixing in the hard plastic material a certain amount of mineral fibres, preferably glass fibres which will increase strength and surface hardness. This is in itself not new as regards the material in that glass fiber reinforced polyester is well known but in the present case there is no reason for using a strengthening reinforcement since the strength of the un-reinforced material will already be quite sufficient for the purpose. The glass fiber intermixing is therefore specifically employed with the object of increasing the surface hardness without impairing the properties which the material should otherwise show precisely for use in wear strips of the mentioned kind.It is thus only to regard as fortunate that there has already for quite different purposes been developed an injection mouldable polyester raw material with a glass fiber content that is also applicable for the present purpose in that it turns out that with such a material it is possible to produce highly advantageous wear strips for co-operation with chains of thermoplastic material irrespective of the fact that the material has been developed for quite different purposes irrelevant to the invention.

It will moreover according to the invention be possible to neutralise the adhesion of foreign particles on the wear strip by intermixing in the material thereof a certain amount of wax or oil, fon example in the form of so-called master bats, which will generally reduce friction also as regards friction between the foreign particles and the wear strip. This wax or oil may just like the mineral fibres be pre-mixed into the granulated material forming the basis of the injection moulding of the wear strip units or these additives may be added by intermixing in the pure plastic granulate.

The mineral fiber intermixing should be between 2% and 65% by weight whereas the wax or oil intermixing should be between 1% and 30% by weight.

The invention will now be further explained in the following with reference to the drawing showing examples of embodiments of the invention wherein Figure 1 shows a first type of conveyor chain with wear strips according to the invention, Figure 2 shows a vertical plane section of the chain and the wear strip in Figure 1, Figure 3 shows another type of conveyor chain supported by a wear strip according to the invention, and Figure 4 shows an example of a wear strip according to the invention where the ends are designed as linking parts.

Figures 1-3 in the drawing show wear strips 1 supporting a conveyor chain 2, for example made of acetal. The wear strip or strips 1 are mechanically supported by track means 3 of steel or aluminium providing the required strength whereas the strips 1 provide the desired friction reduction and wearability. The actual chains 2 and track means 3 are only schematically shown since they do not directly form part of the invention.

Figure 4 shows a top view of a wear strip according to the invention with the ends designed as linking parts 4 and 5 which by snap action may be linked to the next strip in the longitudinal direction.

The snap action link is designed in such manner that forces in the longitudinal direction caused by the friction with the chain may be transmitted between the strips so that it may not be necessary to secure all the wear strips to the track means 3. The arrow in Figure 4 shows the preferred travelling direction of a chain sliding on the strip. Front edge and rear edge of a conveyor track will generally have to be made with a straight edge which may be made by cutting or shearing a strip.

The decisive feature of the invention is that conveyor chains with chain links of thermoplastic materials such as acetal co-operate optimally with plastic wear strips of a surface hard nature, for example of polyester, preferably with addition of an extra hardening fiber material and an extra friction lowering wax or oil material. Particularly by the preferred use of glassfiber but also in general it is moreover important that the wear strips are produced by injection moulding whereby as will be known it is possible to produce a product with a very even surface; also extrusion would be possible but the glass fiber material would be hard on the extruder.By the injection moulding it is possible to make strip units of fairly great lengths, for example 1-1.5 m, and the units may thus in a simple way be directly produced with any required mounting holes and/or snap lock parts,as well as linking parts just as they may be produced with any desired curved courses and profiles which here will not require any further explanation.

Hard plastics such as polyester also has the advantage of being form stabile; by the hitherto for the object developed plastic materials it is even a well-known and more or less accepted drawback that the wear strips stretch by the pull friction from the chain and that this gives rise to special maintenance problems. Also other forms of deformations have been observed and neither stretchings nor twistings will occur to any appreciable extent by the use of polyester or similar materials which moreover are also characterised by not being water absorbing.

By the invention almost all the problems associated with the use of the hitherto developed wear strips are therefore solved. Furthermore, the invention has the extra advantage that the strips according to the invention may generally be designed with smaller thickness than the known strips, for example 1 mm as against 3 mm, which will naturally mean a substantial saving of material. It is moreover mentioned that the wear strip according to the invention is more easily secured by adhesion than the known strips made of softer plastics.

By the injection moulding it is possible to produce even very thin strips, for example with a thickness of about 0.2 mm, and such very thin strips are suitable for securing by adhesion.

Claims (4)

1. Plastic wear strip (1) for chain conveyors particularly for sliding support of conveyor chains (2) having plastic chain links, characterised in that the wear strip (1) consists of a preferably injection moulded material of polyester or a similar surface hard plastic material.

2. Wear strip according to claim 1, characterised in that the material contains mineral fibres, preferably glass fibres to an amount of 2-65 percentage by weight.

3. Wear strip according to claim 1 or 2, characterised in that the material contains 1-30% wax or oil by weight, preferably added in the form of socalled master bats.

4. Wear strip according to claim 1, characterised in that injection moulded wear strip units (1) are made with high surface smoothness and with integral mounting holes, snap lock parts and/or linking parts (4,5).