GB1583162A - U-shaped bar spring - Google Patents

Description

(54) A U-SHAPED BAR SPRING (71) We, VOLKSWAGENWERK AKTIEN GESELLSCHAFT, a German Company, of 3180 Wolfsburg, Hanover, Federal Republic of Germany, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to a U-shaped bar spring for use as a restoring spring for the brake shoes of an expanding drum brake.

Compared with the wire helical springs normally used in expanding drum brakes which, with a relatively low spring volume, enable good distribution of force and pitch and therefore optimum utilization of the working capacity of the spring, U-shaped restoring springs have the advantage that they may be installed without taking up much room in the brake and in particular leaving room for the hydraulic actuating device which acts upon the ends of the brake shoes and possibly for a mechanical auxiliary actuating device. These characteristics are particularly advantageous in brakes of small dimensions. The restoring force of such restoring springs must be neither too high nor too low because on the one hand the brake should respond at a low pressure of the hydrualic fluid and on the other hand even with corroded brakes reliable restoring of the brake shoes should be ensured.

Known U-shaped bar springs (e.g. as disclosed in German Patent Specification No.

1093 227), which generally comprise a wire having a round cross-section which is the same over the entire length of the spring only partially meet the foregoing requirements.

In order to obtain the required working capacity-the previously described restoring force must generally be applied over a spring travel of approximately 7 millimetres (assuming a surface thickness of 5 mm)a relatively great spring volume is necessary in the case of a bar spring having a constant cross-section, so that spring dimensions result which often can only be housed in small brakes with difficulty. A previous remedy was presetting (imparting an initial stress) of the spring, thereby achieving an increase in the working capacity of the spring, i.e a reduction in the required spring dimensions. The disadvantage of this, however, is that the amount of work required to produce such a spring is increased In another generally U-shaped brake shoe return spring disclosed in British Patent Specification No. 944,067, it is proposed to achieve a more uniform stress distribution throughout the entire length of the spring, which may be a leaf spring, by forming the end sections only of the spring as tapered sections of decreasing cross-section in a direction towards their free ends. The major part of the spring intermediate the tapered end sections, however, is of constant crosssection.

An aim of the invention is to improve Ushaped bar springs of the type mentioned above and in particular to construct them in such a manner that, while dispensing with pre-setting of the spring, improved use of the spring material and therefore a reduction in the spring dimensions and a more favourable adaptation of spring force and pitch of spring are achieved.

According to the present invention there is provided a U-shaped bar spring, for use as a restoring spring for the brake shoes of an expanding drum brake, the spring having a cross-section which increases from the free end of each of its arms along substantially the whole length of said arm in such a manner that when the spring is loaded at its ends the bending stresses in the spring at all cross sections are at least substantially equal.

As a result of the selected shape, the working capacity of the bar spring is many times that of bar springs having a constant cross-section. Compared to conventional bar springs, the same applied load produces twice the displacement and the stiffness of the spring is reduced by half. Thus the final force of the restoring spring remains within acceptable limits and even with worm brake linings the contact pressure retains a low value. When manufacuturing the bar spring from strip steel, the bar spring may advantageously be given such a spring contour that the springs are stamped from the bar with the ends of the springs overlapping so that there are only slight cutting losses.

The invention will hereinafter be further described by way of example with reference to the accompanying drawings in which: Fig. 1 shows in diagrammatic form the interior of a drum brake having disposed therin a U-shaped bar spring constructed in accordance with the present invention, and Fig. 2 is a plan view of the bar spring shown in Fig. 1.

Fig. 1 is a diagrammatic illustration of an expanding drum brake whose brake shoes 1 are supported inside the brake drum (not shown in detail). The brake shoes 1 have in the usual manner a T-shaped cross-section and comprise webs 2 and flanges (unnumbered) facing the brake drum and having brake linings fixed thereon. A brake cylinder 4 located in the upper region of the drum brake is connected to the upper ends of the two brake shoes 1 and in this manner transmits the braking force produced by the brake system. When the brake is released, the brake shoes 1 previously pressed by the brake cylinder onto the brake drum are detached again from the brake drum oy means of the restoring spring 3 and are retracted by the so-called air gap. The restoring spring 3 is an at least substantially U-shaped bar spring having a pair of spaced arms connected by a bridge portion, which spring is located at both its ends in corresponding recesses of the shoe webs 2. The cross-section of the bar spring 3 increases from the free end of each of its arms along substantially the whole length of said arm in such a manner that when the spring is loaded at least substantially equal bending stresses arise in all cross-sections of the bar spring.

Fig. 2 shows a particularly favourably constructed and preferred embodiment of the U-shaped bar spring. This bar spring is manufactured from strip steel and has a thickness which increases towards the middle of the spring. Its width is constant over the entire length. Because of its slight width, this bar spring made from strip steel may be easily installed in the drum brake even when spatial conditions are unfavourable. Bar springs made from sheet steel are particularly easy to manufacture since they may be manufactured in a simple stamping process from strip steel. Their contour may be so selected that the stamping process may result in only slight cutting losses. To make a connection between the webs 2 and the ends of the U-shaped bar spring 3, the ends of the bar spring are usually angled off.

This bending, extending in Figs. 1 and 2 somewhat perpendicular to the plane of the page, may of course be done together with the stamping process or directly afterwards in the same device. In contrast to the spring construction shown in Fig. 2 and in order to obtain an even greater spring travel, the spring thickness may be kept constant and the spring width may be increased from the ends towards the middle of the spring. For this purpose, a somewhat trapezoidal cut is selected which is bent over the sheet thickness to form a U.

As a result of the proposed construction of the U-shaped bar springs used in expanding drum brakes, the spring material is utilized to an optimum level and the amount of space required for the spring is consequently reduced. Furthermore, there is the spatial advantage that its restoring force compared with conventional U-shaped bar springs increases substantially less strongly with increasing deflection, this being particularly favourable for their use in drum brakes because the aim here is towards a restoring force which is as constant as possible and as independent as possible of the brake lining wear.

WHAT WE CLAIM IS: 1. A U-shaped bar spring, for use as a restoring spring for the brake shoes of an expanding drum brake, the spring having a cross-section which increases from the free end of each of its arms along substantially the whole length of said arm in such a manner that when the spring is loaded at its ends the bending stresses in the spring at all cross sections are at least substantially equal.

2. A bar spring as claimed in claim 1, which is manufactured from strip steel of constant width and which has a thickness which increases towards the middle of the spring.

3. A bar spring as claimed in claim ], which is manufactured from strip steel of constant thickness and which has a width which increases towards the middle of the spring.

4. A bar spring as claimed in claim 2 or 3, which has a spring contour enabling the springs to be stamped from strip with the ends of the springs overlapping so that there are only slight cutting losses.

5. A bar spring constructed and adapted to operate substantially as hereinbefore particularly described with reference to and as illustrated in the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (5)

**WARNING** start of CLMS field may overlap end of DESC **. The invention will hereinafter be further described by way of example with reference to the accompanying drawings in which: Fig. 1 shows in diagrammatic form the interior of a drum brake having disposed therin a U-shaped bar spring constructed in accordance with the present invention, and Fig. 2 is a plan view of the bar spring shown in Fig. 1. Fig. 1 is a diagrammatic illustration of an expanding drum brake whose brake shoes 1 are supported inside the brake drum (not shown in detail). The brake shoes 1 have in the usual manner a T-shaped cross-section and comprise webs 2 and flanges (unnumbered) facing the brake drum and having brake linings fixed thereon. A brake cylinder 4 located in the upper region of the drum brake is connected to the upper ends of the two brake shoes 1 and in this manner transmits the braking force produced by the brake system. When the brake is released, the brake shoes 1 previously pressed by the brake cylinder onto the brake drum are detached again from the brake drum oy means of the restoring spring 3 and are retracted by the so-called air gap. The restoring spring 3 is an at least substantially U-shaped bar spring having a pair of spaced arms connected by a bridge portion, which spring is located at both its ends in corresponding recesses of the shoe webs 2. The cross-section of the bar spring 3 increases from the free end of each of its arms along substantially the whole length of said arm in such a manner that when the spring is loaded at least substantially equal bending stresses arise in all cross-sections of the bar spring. Fig. 2 shows a particularly favourably constructed and preferred embodiment of the U-shaped bar spring. This bar spring is manufactured from strip steel and has a thickness which increases towards the middle of the spring. Its width is constant over the entire length. Because of its slight width, this bar spring made from strip steel may be easily installed in the drum brake even when spatial conditions are unfavourable. Bar springs made from sheet steel are particularly easy to manufacture since they may be manufactured in a simple stamping process from strip steel. Their contour may be so selected that the stamping process may result in only slight cutting losses. To make a connection between the webs 2 and the ends of the U-shaped bar spring 3, the ends of the bar spring are usually angled off. This bending, extending in Figs. 1 and 2 somewhat perpendicular to the plane of the page, may of course be done together with the stamping process or directly afterwards in the same device. In contrast to the spring construction shown in Fig. 2 and in order to obtain an even greater spring travel, the spring thickness may be kept constant and the spring width may be increased from the ends towards the middle of the spring. For this purpose, a somewhat trapezoidal cut is selected which is bent over the sheet thickness to form a U. As a result of the proposed construction of the U-shaped bar springs used in expanding drum brakes, the spring material is utilized to an optimum level and the amount of space required for the spring is consequently reduced. Furthermore, there is the spatial advantage that its restoring force compared with conventional U-shaped bar springs increases substantially less strongly with increasing deflection, this being particularly favourable for their use in drum brakes because the aim here is towards a restoring force which is as constant as possible and as independent as possible of the brake lining wear. WHAT WE CLAIM IS:

1. A U-shaped bar spring, for use as a restoring spring for the brake shoes of an expanding drum brake, the spring having a cross-section which increases from the free end of each of its arms along substantially the whole length of said arm in such a manner that when the spring is loaded at its ends the bending stresses in the spring at all cross sections are at least substantially equal.

2. A bar spring as claimed in claim 1, which is manufactured from strip steel of constant width and which has a thickness which increases towards the middle of the spring.

3. A bar spring as claimed in claim ], which is manufactured from strip steel of constant thickness and which has a width which increases towards the middle of the spring.

4. A bar spring as claimed in claim 2 or 3, which has a spring contour enabling the springs to be stamped from strip with the ends of the springs overlapping so that there are only slight cutting losses.

5. A bar spring constructed and adapted to operate substantially as hereinbefore particularly described with reference to and as illustrated in the accompanying drawings.