TECHNICAL AREA

The invention refers to a tread for an industrial truck wheel or an industrial truck roll, which tread counteracts the generation of electrostatic charge. The invention further refers to an industrial truck wheel with such a tread as well as an industrial truck comprising such an industrial truck wheel. Finally the invention refers to a method for reducing the generation of static electrostatic charge of an industrial truck wheel or an industrial truck roll.

BACKGROUND Industrial trucks with solid, that is non-air filled, treads become electrostatically charged at driving. The electrostatic charge arises in the composite material of the treads when loading in contact with the floor. The charge is thereafter transferred via the rim to which the treads are attached to the chassis of the industrial truck as well as the electric system of the industrial truck.

In achieving the goal of avoiding electrostatic charge there are on the market so called conductive or anti-static industrial truck wheels respectively. The conductive wheels shall have a resistance below

10 kohm and the anti-static wheels shall have a resistance of between 100 kohm and 10 mohm.

These wheels have treads with added conductive particles in the material, see the document

DE19858825 C2. The individual conductive particles achieve taken together, electric conductive connections between the inner and the outer surfaces of the treads, that is, between the rim and the ground. The connections achieved by the particles could for example be in the form of irregularly running extensions through the tread, see figure of DE19858825 C2. Industrial truck wheels of this type are often required in high explosive risk areas.

The documents US200901078595 Al, EP0819741 A2 and GB544757 describe air filled tires. For the purpose of discharging static electricity from the vehicle that the tires are carrying, an electric conduction path from the vehicle chassis to the ground has been provided.

SHORT DESCRIPTION OF THE INVENTION

When an industrial truck, for example a fork lifter, is driven on conventional industrial truck wheels the chassis of the industrial truck is often charged to a voltage exceeding 10 kV, for example to approximately 14 kV. Tests have disclosed that the charge will reach a maximum when the industrial truck wheels are new, and that the size of the charge also varies with the surrounding assumptions as wheel loading, type of ground, and air humidity. High values of charge occur for example when driving on a concrete floor having an electrically isolating plastic coating, such as an epoxy coating. Further the value of charge is decreasing with increasing air humidity, and is increased with increasing wheel loading.

Charge over 8 - 10 kV is not desirable as it can lead to sparks, discomfort, component damages as well as function disturbance.

The pivoted wheels described in DE19858825 C2 have the disadvantages of, compared with conventional industrial truck wheels, being relatively expensive, give a less bearing capacity and have a shorter endurance.

The goal with the present invention is to in a cost efficient manner reduce the electrostatic charge that can occur at a chassis of an industrial truck, without deteriorating the performance of the industrial truck. The goal has been achieved by providing a solid tread for an industrial truck wheel or an industrial truck roll, where at least one of the side surfaces of the tread has an electrically conductive coating, which reduces generation of electrostatic charge in the tread.

The tread has the shape of a cylinder and its side surfaces are defined as the cylinder's both circular ends. Through the invention the electrostatic charge at an industrial truck chassis is limited, which wheels of the industrial truck is provided with the inventive treads, to 2,5 - 5 kV. A charge within this range does not give rise to any of the above mentioned problems.

The electrically conductive coating on the tread side surface suppresses the occurrence of electrostatic charge. Through this the invention has the desired effect also when driving on electrically isolated grounds, for example plastics such as epoxy. Other advantages are that the tread only implies a marginal cost increase and does not affect the driving performance or the admissible c

loading. Furthermore the effect of the invention is predictable as it is essentially not affected by varying air humidity. Furthermore the invention can be applied on existing conventional industrial truck wheels by applying the coating. The invention has its best effect if the coating covers the entire tread side surface. The invention is however functioning to satisfaction even if the coating does not cover the entire side surface, which for example can be the case after the wear that occurs after a time period of operation.

The coating can be applied such that it covers the entire side surface of the tread and also, thereby electrically contacts, overlaps the rim that the tread is mounted on. Further the coating can extend around the edge of the side surface to the outer surface of the tread, which is the side that in operation is facing the ground. In a corresponding manner the coating can be applied to the inside of the tread, which is the side that in operation is facing the rim.

A simple and cost efficient manner to provide the tread with the electrically conductive coating is to cover both the tread side surface and the rim side surface with the coating. Through this is additionally achieved god electric contact between the tread and the rim.

The invention functions at its best if the both of the side surfaces of the tread are covered with the electrically conductive coating. The effect of the invention can however also be achieved by coating only one of the side surfaces, which facilitates the production of the tread as only one side surface needs to be treated.

If the electrically conducting coating is of a colour that sharply deviates from the colour of the tread, it is possible to visually in a simple manner determine that an industrial truck is provided with the tread and by this is not heavily electrostatically charged. This is of advantage in operations having certain areas to which only vehicles with low charge are permitted. In addition the deviating colour serves as a distinct marking when handling wheels before mounting on an industrial truck.

The tread can be made in polyurethane which implies good admissible loading and good driving performances. The effect of the invention can however in principal always be reached with a tread from other materials, such as rubber.

The goal of the invention has further been achieved through an industrial truck wheel or an industrial truck roll that has the said tread, and through an industrial truck with such an industrial truck wheel or such an industrial truck roll. The invention functions at its best if all wheels and/or rolls are of the type according to the invention.

The goal of the invention has finally been achieved by providing of a method to reduce generation of electrostatic charge at an industrial truck wheel or an industrial truck roll when loaded in connection with driving of an industrial truck that is carried by the industrial truck wheel or the industrial truck roll. The method comprises the step of coating at least one side surface of the solid tread of the industrial truck wheel or the industrial truck roll with an electrically conducting coating. Through this the invention can be applied to existing wheels.

DESCRIPTION OF FIGURES

In the figures an embodiment of the present invention is disclosed. In which is disclosed figure 1 a plane view and a side view of an industrial truck wheel with a tread according to the present invention, figure 2 side views from further embodiments of the present invention, and figure 3 a comparison in static charge when using a conventional wheel and a wheel

according to the present invention respectively on different grounds.

DESCRIPTION OF EMBODIMENTS

Figure 1 shows in a schematic manner an industrial truck wheel 10 or an industrial truck roll 10. The difference between an industrial truck wheel and an industrial truck roll is mainly that an industrial truck wheel is larger and designed to carry a larger load. An industrial truck wheel often operates as a powered and steering wheel, while an industrial truck roll often operates as a support leg wheel. In the following description only industrial truck wheels are discussed, the invention however is functions in the same manner with an industrial truck roll.

On the left side of figure 1 the industrial truck wheel 10 is shown in a plane view. The industrial truck wheel 10 has a rim 15 to which a tread 20 is mounted. On the right hand side of figure 1 the same wheel 10 is shown seen in the travel direction. The tread has the shape of a cylinder, more precisely a hollow cylinder. The in cross section circularly shaped tread 20 of the wheel 10 has a first side surface 20a and a second side surface 20b. To the first side surface 20a of the tread is a first coating 30a applied. The first coating 30a covers the entire first side surface 20a of the first tread. The corresponding is valid for the second side surface 20b of the second tread, where a second coating 30b has been applied.

Figure 2 discloses different variants when it comes to the coating of the tread 20 and the rim 15 seen in the direction of travel.

At far left in figure 2 a wheel 10 is illustrated with different coating on the first side surface 20a of the tread 20 and the second side surface 20b respectively. The first coating 30a covers the entire side surface 20a of the tread and also overlaps the rim 15. The second coating 30b covers the entire second side surface 20b of the tread and also the entire rim 15.

In the middle of figure 2 a wheel 10 is illustrated that is only covered on one side surface. The coating 30a extends over the entire first side surface 20a and around the peripheral outer edge to the outer surface 20c of the tread, which is the surface that is facing the ground.

The embodiments to the left and in the middle of figure 2 illustrate that different coatings can be utilized on the different sides of the tread and/or the rim.

To the right of figure 2 is illustrated a further alternative for the coating 30a, 30b. Here again, as in figure 1, the same coating is used on the first and second side surfaces of the tread 20. The coatings 30a, 30b extend over the entire first and second side surfaces and around their peripheral outer edges to the outer surface 20c of the tread. The coatings 30a, 30b are further applied on the inner surface 20d of the tread, which is the side which is facing the rim 15.

To summarize, figures 1 and figures 2 illustrate several alternatives when considering the extension of the coating on the wheel. The coating can be applied exclusively to the tread, or on the tread and the rim. The coating can be of the same type on both sides, or be of different type on the sides.

In the figures the thickness of the coating is exaggerated for the purpose of clarifying the invention.

As already described the electrostatic charging that an industrial truck is exposed to varies, depending on the ground that the industrial truck is driven on. In figure 3 the charge when driven on different grounds A - D. Figure 3 discloses the result of real measurements, where all wheels 10 of an industrial truck have carried treads with an electrically conducting coating 30a, 30b on both sides 20a, 20b of the tread, see figure 1. In each case A - D are disclosed a dashed bar and a non-dashed bar. The dashed bars represent the charge that arises when operating wheels 10 according to the present invention, while the other bars represent ordinary wheels. As seen the wheels according to the present invention have a favourable effect on each of the grounds A - D. More in detail the bars A concern the electrostatic charge that arises when operating on a first ground. Here the wheels of the invention bring about that the charge has been reduced by approximately 55 %. The bars B represent results measured when operating on a second ground, where the charge is reduced by 75 %. C represents operating on a third ground and a reduction of approximately 90 % has been established. Finally the bars D represent operating on a fourth ground and show a reduction of approximately 70 %. To sum up the wheels of the invention implies that the electrostatic charge has been reduced by 55 - 90 % compared to conventional industrial truck wheels.

As has been mentioned charging over approximately 8 - 10 kV is not desirable. Figure 3 discloses that cases B, C and D imply charge in this interval, if conventional industrial truck wheels are used. In all cases the usage of the wheels of the invention has reduced the charging to an acceptable level.

The invention implies coating of a tread with an electrically conductive coating. The coating can for example be Graphite 33 from Kontakt Chemie. According to the method of the invention the coating can for example be sprayed or painted on the tread, or on the tread and the rim that carries the tread. Through this the coating can, according to the embodiments above, be applied to one or both side surfaces of the tread. The method can additionally comprise coating of the inner and/or outer surfaces of the tread.