Abstract
The invention relates to a brake shoe for a drum brake and a drum brake that includes a supporting unit and a lining unit, wherein the supporting unit comprises a bearing portion for pivotable support and an actuation portion, and wherein the supporting unit has a bending stiffness, which has different values between the bearing portion and the actuation portion.
Claims
1. A brake shoe for a drum brake, comprising: a supporting unit, which comprises a bearing portion for pivotable support, an actuation portion, a first supporting portion adjacent the bearing portion and a second supporting portion adjacent the actuation portion; and a lining unit having a first lining portion and a second lining portion where a majority of the first lining portion is positioned adjacent the first supporting portion and a majority of the second lining portion is positioned adjacent the second supporting portion; wherein the supporting unit has a bending stiffness that increases from the bearing portion toward the actuation portion such that the second supporting portion has a greater bending stiffness than the first support portion; wherein the first and second supporting portions of the supporting unit comprise a web plate and a lining plate, wherein the at least one of the web plate and the lining plate of the first and second supporting portions has at least one of a cross-section area and a surface which increases from the bearing portion towards the actuation portion; and wherein when an actuation force smaller than a limiting force is applied to the actuation portion the lining unit shows a braking effect in the first lining portion only, and when an actuation force larger than the limiting force is applied the lining unit shows a braking effect also in the second lining portion.
2. The brake shoe of claim 1, wherein the at least one of the cross-sectional area and the surface comprises a material gauge.
3. The brake shoe of claim 1, wherein the supporting unit comprises at least one local material weakness, which reduces the bending stiffness of the supporting unit.
4. The brake shoe of claim 1, wherein the actuation portion comprises a first engagement portion and a second engagement portion, wherein the first engagement portion is designed to transmit an actuation force smaller than a limiting force to the supporting unit, and wherein the second engagement portion is designed to transmit an actuation force larger than the limiting force to the supporting unit.
5. The brake shoe of claim 3, wherein the first lining portion comprises a material having a greater abrasion resistance than the second lining portion.
6. The brake shoe of claim 3, wherein the second lining portion comprises a material having a higher coefficient of friction than the first lining portion.
7. The brake shoe of claim 6, wherein the coefficient of friction of the second lining portion is within the range of from about 1.1 to about 3 times the coefficient of friction of the first lining portion.
8. The brake shoe of claim 7, wherein the coefficient of friction of the second lining portion is within the range of 1.15 to 2 times the coefficient of friction of the first lining portion.
9. The brake shoe of claim 8, wherein the coefficient of friction of the second lining portion is within the range of 1.3 to 1.7 times the coefficient of friction of the first lining portion.
10. The brake shoe of claim 1, wherein the lining plate includes at the first lining portion a first thickness that is proximate the bearing portion and at the second lining portion a second thickness that is proximate the actuation portion.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
(1) FIG. 1 a side view of a preferred embodiment of the brake shoe according to the invention,
(2) FIG. 2 a first sectional view of a preferred embodiment of the brake shoe according to the invention,
(3) FIG. 3 a second sectional view of a preferred embodiment of the brake shoe according to the invention,
(4) FIG. 4 a view of the drum brake according to the invention in the first state,
(5) FIG. 5 a view of the brake drum according to the invention in the second state,
(6) FIG. 6 a view of a preferred embodiment of the actuation portion according to the invention, and
(7) FIG. 7 a view of a preferred embodiment of the brake shoe according to the invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
(8) The embodiment of the brake shoe 1 according to the invention shown in FIG. 1 comprises a supporting unit 2 and a lining unit 4. The supporting unit 2 preferably is formed arc-shaped or curved, wherein the lining unit 4 is arranged at the convex outside of the curvature. The supporting unit 2 comprises a bearing portion 23 and an actuation portion 24, wherein the bearing portion in the preferred embodiment shown is a bore, for example, by means of which the supporting unit 2 may be pivotably mounted by means of a pin, for example. Furthermore, the supporting unit 2 comprises a first supporting portion 21 and a second supporting portion 22, wherein the first supporting portion 21 preferably has a lower bending stiffness than the second supporting portion 22. The lining unit 4 preferably in the area of the first supporting portion 21 comprises a first lining portion 41, and in the area of the second supporting portion 22 it comprises a second lining portion 42. Particularly preferably, as is shown, the thickness D of the lining unit in the area of the first lining portion is on average larger than in the area of the second lining portion 42.
(9) FIG. 2 shows the sectional view indicated by reference numeral II in FIG. 1 of a preferred embodiment of the brake shoe according to the invention. The T-shaped design of the supporting unit 2, which comprises a lining plate 28 and a web plate 27, is clearly shown. The brake shoe is shown as a sectional view in the area of the first lining portion 41 or of the first supporting portion 21, respectively, and the lining unit 4 has a mean thickness D.sub.1 in this area. Preferably, the supporting unit 2 may also have a double-T-shaped cross-section, i.e. two parallel web plates 27, or be box-shaped, for example, wherein the web plate 27 is U-shaped and connected to the lining plate 28 with its distal ends.
(10) FIG. 3 shows the sectional view of the brake shoe indicated by reference numeral III in FIG. 1. The lining unit 4 is shown in sectional view in the second lining portion 42 or in the area of the second supporting portion 22, respectively, and has a thickness D.sub.2, which is smaller than the thickness D.sub.1 of the first lining portion 41 (cf. FIG. 2). Furthermore, it is apparent that the lining plate 28 of the supporting unit 2 in the area of the second supporting portion 22 has a larger thickness or material gauge than the lining plate 28 in the area of the first supporting portion 21. Consequently, the lining plate in the area of the second supporting portion 22 has a greater bending stiffness than the lining plate 28 in the area of the first supporting portion 21. The same applies to the web plate 27, which in the area of the second supporting portion 22, as is shown in FIG. 3, has a greater material gauge or wall gauge or plate thickness than in the area of the first supporting portion 21, as is shown in FIG. 2.
(11) FIG. 4 shows an embodiment of the drum brake according to the invention in its first state. The drum brake comprises a brake drum 10 and a brake shoe 1, wherein the brake drum 10 preferably is a brake drum as it is known from the prior art and is only partially shown in the Figure. The brake drum 1 comprises a supporting unit 2, which is pivotably mounted in its bearing portion 23 and to which an actuation force F is transmitted by an actuation element in an actuation portion 24 of the supporting unit 2. The bearing portion 23 of the supporting unit 2 is preferably designed as a ball joint, so that the brake shoe 1 is pivotable relative to the brake drum 10 not only orthogonally to the preferably cylindrical inner surface of the brake drum 10, but also about a radial axis running through the bearing portion 23. Advantageously, the brake shoe 1 may in this way by means of slight tilting or inclining movements compensate for deviations from the cylinder shape, which may occur at the brake drum 10 due to heating, for example. The brake drum 10 preferably rotates in the rotation direction R shown relative to the brake shoe 1, towards the bearing portion 23, so as to be able to make use of the self-servo effect of the drum brake. The first state of the drum brake is characterized in that an actuation force F is applied at the actuation portion 24, which is smaller than a limiting force F.sub.G, so that the brake shoe 1 and in particular the lining unit 4, which is not shown in this Figure, comes to rest against the brake drum 10 in its first lining portion 41 only. In this context, the limiting force F.sub.G is to be understood as an abstract parameter, which results from the constructive design of the drum brake. It may also be defined as the value of the actuation force F, at which the brake shoe 1 only just rubs against the brake drum 10 in the first lining portion 41 thereof. When there is provided a plurality of lining portions 41, 42, 43, there will consequently be a first limiting force F.sub.G1, up to which only the first lining portion 41 rubs, a second limiting force F.sub.G2, up to which only the first and the second lining portions 41, 42 rub, and so on. The transition between the lining portions 41, 42, 43 may be stage-less in case leaps are to be avoided in the course of the brake force when the actuation force increases continuously. Alternatively, the individual lining portions 41, 42, 43 may also be clearly delimited against each other, for example by means of grooves, cavities or gaps between the lining portions 41, 42, 43. In order to influence the bending stiffness of the brake drum 1, the supporting unit 2 has local material weaknesses 29, which by way of example are formed as cavities and recesses.
(12) FIG. 5 shows the drum brake already shown in FIG. 4 in its second state. The actuation force F is larger than the limiting force F.sub.G, and the brake shoe 1 rests against the brake drum 10 with more contact or friction surface than the first lining portion 41. The state shown in FIG. 5 is preferably that state, in which the actuation force F has reached its maximum, i.e. the brake shoe 1 rubs against the brake drum with maximum friction surface. The local material weaknesses 29 make it possible that the supporting unit 2 and, consequently, the entire brake shoe 1 in reaction to the actuation force F bends in the desired manner. Thus, it is possible to purposefully cause only a certain lining portion 41, 42, 43 to come into friction with the brake drum 10 for a certain actuation force F.
(13) FIG. 6 shows a preferred embodiment of the actuation portion 24. Preferably, the actuation portion 24 has a first engagement portion 241 and a second engagement portion 242, which are each formed two-dimensional or planar and which are pivoted relative to each other. Below the actuation portion 24, there is indicated an actuation element (without reference sign), which transmits an actuation force F to the actuation portion 24. The continuous lines in the Figure show how, in the first state of the drum brake, the actuation element transmits an actuation force F smaller than a limiting to F.sub.G to the first engagement portion 241. It becomes apparent that by diagonally arranging the first engagement portion 241 the resultant force acting on the supporting unit 2 has a force component directed towards the right. The dashed lines in FIG. 6 show the preferred position of the supporting unit 2 in the second state of the drum brake, wherein the actuation element acts on the second engagement portion 242. Preferably, the actuation force F is larger than the limiting force F.sub.G, wherein the force actually transmitted to the supporting unit 2 has another main direction than the actuation force transmitted to the supporting unit 2 in the first state of the drum brake. In this preferred embodiment, in the first state, a force is transmitted to the supporting unit 2, which force acts at least in certain areas along the circumferential direction of the supporting unit 2 or the brake drum 10 surrounding said supporting unit 2 (cf. FIGS. 4, 5). Preferably, the first lining portion (not shown in FIG. 6) in the first state of the drum brake is forced against the brake drum with an additional force. As a matter of course, the first engagement portion 241 and the second engagement portion 242 may also be formed rounded, wherein the transition between the two engagement portions may also be formed rounded. Preferably, the actuation portion 24 is adapted to the geometry of the actuation element in order to achieve an optimum transmission of force from the actuation element to the supporting unit 2.
(14) FIG. 7 shows a further preferred embodiment of the brake shoe 1 according to the invention. The brake shoe 1 preferably comprises a supporting unit 2 with three supporting portion 21, 22, 25. The supporting portion 21 adjacent to the bearing portion 23 has a mean curvature, the center M.sub.1 of which is preferably displaced relative to the centers M.sub.2 and M.sub.3 of the mean curvatures of the second supporting portion 22 and of the third supporting portion 25. By way of example the distance a of the center M.sub.1 from the center M.sub.2 is shown, which preferably is in a relationship of 0.01 to 0.2, more preferably 0.05 to 0.1, and particularly preferably 0.07 to 0.09 to the mean radius of curvature of the first supporting portion 21. Furthermore, two of the supporting portions 21, 22, 25 preferably have local material weaknesses 29, which influence the bending stiffness of the supporting unit 2 along the course of curvature between the bearing portion 23 and the actuation portion 24. Preferably, there are provided material weaknesses 29, as recess for example, also in the transitional area between two supporting portions 21, 22, 25 so that the supporting portions 21, 22, 25 may be easier pivoted relative to each other and it is possible to achieve that the brake shoe 1 rests optimally against the brake drum 10 (not shown) during different operational states of the brake. FIG. 7 shows also the preferred design of the lining portions 41, 42, 43, wherein they are made from different materials. Advantageously, the first lining portion 41 has a higher abrasion resistance than the second lining portion 42 and the third lining portion 43, wherein the third lining portion 43 in turn has the highest coefficient of friction of the lining portions 41, 42, 43. Thus, the brake and wear behavior of the brake shoe 1 for certain expected times of stress of the individual lining portions 41, 42, 43 may be optimized during operation and for the required brake forces. In the embodiment shown, all lining portions 41, 42, 43 have the same thickness. Alternatively, it could be preferred that the thickness D.sub.1 of the first lining portion 41 is larger than the thickness D.sub.2 of the second lining portion 42, and the latter in turn is larger than the thickness D.sub.3 of the third lining portion 43.
LIST OF REFERENCE SIGNS
(15) 2supporting unit 4lining unit 10brake drum 21first supporting portion 22second supporting portion 23bearing portion 24actuation portion 25third supporting portion 27web plate 28lining plate 29material weakness 41first lining portion 42second lining portion 43third lining portion 241first engagement portion 242second engagement portion D.sub.1thickness of the lining unit adistance D.sub.1first thickness D.sub.2second thickness Factuation force F.sub.Glimiting force M.sub.1, M.sub.2, M.sub.3center of the mean curvature Rdirection of rotation
