CS219890B2 - Brake key for brake control with internal jaws - Google Patents

Abstract

The invention relates to a 'brake key for operating a brake' with internal jaws, comprising a brake drum and brake shoes which can be spread by means of a brake key against return springs, where the brake key is formed by a symmetrical S-shaped expansion cam with a 'camshaft and convex axial involute working surfaces. The essence of the invention is that the involutes of the working surfaces are derived from mutually offset basic circles of the same diameter.

Description

(54) Brake key for brake control with internal shoes

BACKGROUND OF THE INVENTION The present invention relates to an inner shoe 'brake key' comprising a brake drum and brake shoes which can be expanded by means of a brake key against return springs, the brake key being a symmetrical S-shaped cam with a camshaft and convex axial involute workspaces.

The essence of the invention is that the involutes of the work surfaces are derived from offset bases of the same diameter.

The invention relates to a brake key for actuating a brake with internal jaws.

As is known, existing brakes with internal brake shoes consist of a brake drum in which the brake shoes are mounted, which can be expanded by a brake key against a white return spring. The brake shoes are pivoted on one end of the brake shield pin while the other ends of the brake bearings are generally formed by a symmetrical S-shaped expansion cam with which the camshaft is connected to the spacer cam When the brake shoes are disengaged with the brake key, the brake linings abut on the inner surface of the brake drum and, by friction, produce a braking effect which is the greater the A brake with internal (jaws of this type is described, for example, in DT-AS 1 455 788. A constant ratio between the rotation of the brake key and the braking force is a basic requirement for braking in general, especially for vehicle brakes, where the same movement then the brakes always produce the same braking effect, so a slight mental concentration is required when driving, which is particularly important for high-performance vehicles such as trucks and buses. But this desirable stability or proportionality cannot be achieved; is With respect to the performance characteristics of the brake lining and its counterpart. Adjustments eliminating the effect of wear on the contact surfaces in order to achieve the original braking force-to-braking torque ratio appear to be inadequate, since not only the dimensions change but also the physical properties of the contacting parts are subject to changes caused by frequent mechanical and thermal stresses.

It is an object of the invention to overcome this drawback.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a brake key in which a constant ratio between the movement of the brake and the braking force can be achieved by means of a brake shoe with internal shoes. In solving this task, it is based on the familiar; the brake key, which consists of a symmetrical S-shaped cam with camshaft and convex axial evolutionary working surfaces.

In the known brake keys of this type, the two involute working surfaces are derived from one common base circle whose center lies on the cam axis; shaft. Consequently; When turning the brake key, the arms on which the forces act on the ends of the brake shoes do not change, so that the spacing forces also remain constant. Then, however, the braking effect does not remain constant on the basis of the already mentioned substrates, the change in the ratio between the brake rotation; keys and braking force increases with the number of brakes.

In order to compensate for this change and thus create a constant ratio between the movement of the brake key and the braking effect, it is necessary to vary the discrepancy force as a function of the rotation of the brake key. It is known that this contradictory dependence between the conflicting forces and the rotation of the brake key can be achieved if the involute working surfaces of the spacer cams do not have one common base circle, but instead are derived from two separate base circles offset from each other. The centers of the two basic circles lie on the circumference of the central circle whose center lies on the camshaft axis. The centers of the basic circles lie on the circumference of the central circle opposite each other within the angular sector defined by the involutes of the work surfaces. When the camshaft is then rotated, the Force arm changes so that the expansion force changes accordingly so that a constant ratio between the expansion force and the cam rotation can be achieved; shaft, which practically means the achievement of a direct braking characteristic envisaged by the invention.

As will be explained in more detail below, the lengths of the arms of the force during rotation of the spreader cam mění vary substantially from maximum to minimum, but there may be some initial range in which the arm lengths will increase when the cam is rotated. Thereafter, the alignment considered will be even more perfect since it also takes into account the initial increase in braking efficiency.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a cross-sectional view of a brake actuated spacer cam with a spacer cam according to the invention; FIG. 2 is a perspective view of a spacer cam according to the invention; 6 shows various operating positions of the spacer cam according to the invention.

As shown in the drawing, bbr. 1, the brake drum 10 and the shaft 12 with the brake shield 14 are arranged in a conventional manner. The brake shield 14 carries two opposing half-moon brake shoes 16, 18 which are rotatably supported on one end (pin 20 and 22 respectively). The opposite ends of the brake shoes 16 and 18 respectively carry rollers 24 and 26, respectively. The brake shoes 17 and 18, respectively, are provided with brake linings 28 and 30, respectively, abutting the cylindrical inner surface of the brake drum 10, while the pulleys 24 and 18, respectively. 26, the cams 32 are movable by means of which they can be expanded against the tension springs in the usual manner, for the sake of clarity these tension springs are not shown in the drawing.

Details of the spacer cam 32 are shown in FIG. 2. The spacer cam 32 has an elongated cam shaft 34 with a longitudinal Z axis in the axial direction. The cam shaft 34 carries two transverse bent lever arms 36, 38 arranged opposite to each other and provided with convex axial working surfaces 40 , respectively. 42, wherein the irivolutes or evolvents of the work surfaces 40 and 42 are respectively inverted. The work surfaces 40, 42 define the angular range alpha relative to the longitudinal axis Z between the first boundary line A1 and the second boundary line A2. The boundary lines A1, A2 extend on one side by the longitudinal axis Z at the center C and on the other side by the initial and the second axes respectively. eivolvent II, 12, as will be shown below.

According to the invention, the involts II, 12 are identical but are assigned to the two basic circles E1 and 12 respectively. B2, whose centers C1, C2 are opposite one another on the periphery of one central circle B, which is concentric with the center C and the longitudinal axis Z.

The centers C1, C2 define a control line Ac which, together with the first limit line AI, closes the control effort ac and extends within the angular range and the longitudinal axis Z. The control angle ac is at least as large as the reference angle ar which is closed by the limit line AI and reference line Ar. The reference line Ar also extends through the center C as well as the center of curvature C3 of the involute II, to which the intersection of said involute is associated with the first oil line A1,

FIG. 3 shows the meaning of an embodiment of the spacer cam according to the invention. The graph shows the dependence of the braking torque M on the number N of braking. The braking torque M of the new brake or new · lining shows the initial value Mo. As a rule, the braking torque during the first braking increases until the contact surfaces between the brake lining and the brake drum have been fully matched. This running-in time lasts until the maximum value Mroax of the braking torque is reached after the number of braking NI. This increase in braking torque can only be compensated if the arm of the spreading force is longer, since the spreading forces then suitably decrease at a constant braking torque. For further braking operations over N1, the braking torque is continuously reduced until the minimum value Mmin of the braking torque is reached at the number of N2 braking. To compensate for such a loss, the force arm must again be reduced, the force itself increasing and the braking torque unchanged.

Figures 4 to 6 illustrate the geometry of the various relative positions of the spreader cam and the force transmission pulleys.

As is known, each involute point is associated with one center of curvature that lies on its side on the circumference of the associated base circle from which the involute is derived. The center of curvature determines the radius of curvature with the corresponding involute point. The radius of curvature is tangential to the circumference of the base circle. In the illustrated embodiment, the involute II is derived from the base circle B1 and the involute 12 is derived from the base circle B2. Their centers Cl, respectively. C2 lies on the periphery of the central circle D 'opposite one another.

FIG. 4 shows the relative starting position of the spreader cam and the pulleys. As is evident, the boundary lines A1, A2 pass through the center C as well as the first point P1 and the last point U2 of the involute II. This also applies to the involute 12. The inverse point P1 of the involute II is assigned a center of curvature C3 Bl. In the same way, the end point P2 of the involute II is assigned the center of curvature C4, also lying on the base circle B1.

It is further evident that the centers C1 and 2, respectively. C2 of the basic circles B1, respectively. B2 defines a control line Ac which extends through the center C 'and diametrically intersects the two base circles B1, B2. The control line Ac closes the control angle ac with the first limit line A1. On the other hand, the center C and the center of curvature C3 determine the reference line Ar, which closes the reference angle ar with the first limit line A1. As can be seen, in the illustrated embodiment, the control angle ac is greater than the reference angle ar. These mutual dimensional relationships ensure that when the expansion cam is rotated about its longitudinal axis Z (according to FIG. 2) in the clockwise direction, the arms of the expansion force initially increase, so that the actual expansion forces decrease temporarily, so that the braking torque is achieved. as already explained by the diagram in Fig. 3.

In the initial position shown (FIG. 4), the involute-shaped convex work surface 40 (FIG. 2) meets the roller 26 in an axial line that passes through P1. Expansion force, which at this point affects. the roller 26 passes through the center of curvature C3 and its value depends on the distance of the radius of curvature C3 - Pl from the center C1. wherein C3-C represents the arm of the expansion force that is propelled to the throw of the involvent P1 and is denoted by r _o .

When the spreader cam rotates in relation to the image in a clockwise direction, the spreader arm length will increase until the control line A assumes the position in which the lever arm · 36 and / or the spreader arm. 38 comes into contact with the pulleys 24, respectively. 26 point P3 resp. P4 involves II, resp. 12, as shown in FIG. 5. Thereafter, the arms of the spreading force are formed by the distance between the center C and the center of curvature CS, respectively. C6, and 'are indicated. r _max . The arms of the spreading force therefore pass not only through the center C, but also through the centers C1, C2 of the basic circles B1, respectively. B2 along a line on which the diameters of these circles lie in alignment.

As the spreader cam 32 continues to rotate in the present sense, the relative position of the spreader cam 32 and the rollers 24, 26 shown in FIG. 6 will eventually be achieved. In this position, the involute II touches the circumference of the roller 26 'at the throw P2 of the involute. The expansion force acts on the arm r _min . Similar conditions apply to lever arm 38 and roller 24.

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As seen, arm razpěrných force is changed so that upon rotation of the expansion toggle 32 from the initial angular position of FIG. 4 via an intermediate position according to FIG. 5 to · the angular position of FIG. 6 initially increasing and is increased from the values r ₀ and to · maximum r _max , then decrease to r _min . This change is evident from the fact that, with the exception of the opposing positions shown in FIG. 5, the arms of the restraining forces lie on the cutting edges of the base circles B1, B2 and thus necessarily shorter than in the position of FIG. .

When designing a spacer cam according to the invention, the dependence of the drum brake of the selected type with the conventional spacer cam will first be investigated. Then the line of maximum value M _max assigned to the number of braking NI forms the base or the base position for selecting the different angles ac and ar, the values of which will suitably lie within the following ranges.

890 a (c - ar = 15 ° to 35 ° a = 9 ° to 12 ° c and = 60 ° to 90 °.

The diameter of the central circle D will expediently be

D = 2 to 10 mm.

As can be seen, the spacer cam according to the invention differs from the conventional cam only by the geometry of its involute surfaces, so that despite its progress, its acquisition costs remain practically the same.

By the above explanation, the invention has been elucidated on the basis of a spreader cam where the variations in braking torque are also compensated in the range between NO and NI. However, when requirements are milder or the manufacturing process requires it, alignment in such an initial range may also be omitted. Then ac is equal to ar.

Claims (3)

Subject A brake shoe with inner shoe control, comprising a brake drum and brake shoe, which can be expanded by means of a brake key against return springs, the brake shoe comprising a symmetrical S-shaped cam with camshaft and convex axial involute work surfaces, in that the involutes (II, 12) of the work surfaces (40, 42) are derived from mutually offset base circles (B1, B2) of the same diameter. Brake key according to claim 1, characterized in that the involutes (II, 12) enclose each one angular range (a) between the first limit line (AI) and the second limit line (A2), which both pass through the center (C) of the central circle (D) lying on the axis (Z) of the camshaft invention (34), wherein two points lying opposite each other on the diameter of the central circle (D) form the centers (C1, C2) of the base circles (B1, B2) and define the control line (Ac) ) given by the angular range (a) determined by the involutes (II, 12), while the control line (Ac) forms a control angle (ac) with the first limit line (AI), and the points (P1) lying on the first limit line (AI) The curvature (C3) of the involute (II, 12) which defines a reference line (Ar) enclosing a reference angle (ar) with the first limit line (AI) that is at most as large as the control angle (ac). 3. The brake key according to claim 2, wherein the control angle (ac) is greater than the reference angle (ar).