This invention relates to a drum brake device which functions as a leading-trailing (LT) type when the service brake is operated and functions as a duo-servo (DS) type when the parking brake is operated. More specifically, the invention relates to an improvement in a drum brake device to improve the operation feeling of the parking brake actuator.
For example, this type of drum brake device is disclosed in Australian Patent Number AU-B1-53 491/70 and in U.S. Pat. No. 5,275,260. Both conventional drum brake devices are basically the same in their fundamental function, and they are explained with reference to FIG. 10.
The drum brake device in FIG. 10 comprises a pair of brake shoes b, c mounted on a back plate a, an anchor block d provided between respective ends of the pair of brake shoes b, c, a hydraulic cylinder g provided between respective other ends of the pair of brake shoes b, c, a parking brake lever j pivoted at one end i of one brake shoe b, an idle lever k oscillatably pivoted on the other brake shoe c, and rods l, m provided between both brake shoes b, c, wherein one end n of rod l engages the parking brake lever j and the other end o engages the idle lever k, one end p of rod m engages one brake shoe b and the other end q engages the other brake shoe c and the idle lever k.
Operation of the above-described drum brake device is explained next.
In service brake operation, when the hydraulic cylinder g is pressurized, both brake shoes b, c open with the point of abutment against the anchor block d as the fulcrum and frictionally engage with the brake drum, not shown in the diagram, thereby braking as a LT type brake.
In parking brake operation, when the parking brake lever j is pulled in the direction of arrow X, an operational force is transferred to the one rod l, the idle lever k, and the other rod m in order, thereby opening one brake shoe b to frictionally engage with the brake drum. Then, the idle lever k moves away from a center of the brake drum device such that a point of abutment of the idle lever k is against the rod m as the fulcrum, and the pivot point of the idle lever k presses the other brake shoe c in the direction of Y to frictionally engage with the brake drum. Further, a reaction force of the parking brake lever j in the direction of arrow Z affects on one end i of one brake shoe b.
If at this time, a rotational force is applied to the brake drum in the direction of arrow R as when the vehicle is stopping on an uphill or a downhill slope, the friction force of one brake shoe b is transferred to the other rod m, whereby its other end q presses against the other brake shoe c, supported by the anchor block d, in a duo-servo braking action. If the rotational force is applied to the brake drum in the direction opposite to the direction of arrow R, the friction force of the other brake shoe c is transferred to the other rod m, whereby its one end p is pressed against brake shoe b, supported by the anchor block d, in the same duo-servo braking action as above.
As is evident from this parking brake operation, if the other end q of the other rod m abuts against the idle lever k and a clearance exists between the other end q and the other brake shoe c, then when the shoe c rotates in the direction opposite to the direction of arrow R, the position of the hydraulic cylinder g is repelled to the amount equivalent to the clearance. That is, the brake pedal is repelled which is not only disconcerting to the driver, but the pedal stroke increases in the next brake pedal application. Conversely, if the end q of the other rod m abuts against the other brake shoe c and a clearance exists between the other end q and the idle lever k, then the stroke of the parking brake lever j increases by an amount equivalent to this clearance; that is, the stroke of the hand lever increases. From these perspectives, it is preferable that the clearance between the other end q of the other rod m and either the other brake shoe c or the idle lever k be as small as possible.
FIG. 11 illustrates the concept of an automatic shoe clearance device which is installed in the drum brake device of U.S. Pat. No. 5,275,260. A bent end y of an adjustment lever r is pivotable on the web of the brake shoe c. One end of an upper arm s is connected to a groove of an upper strut t for engagement therewith. Another arm is connected to a star wheel u of the upper strut t. A spring w, stretched between the adjustment lever r and the pivot lever v, energizes the adjustment lever r in the counterclockwise direction, with the end y as the fulcrum.
Should the brake lining be worn causing the two brake shoes b, c to open by more than a prescribed value when the service brake is applied, the upper arm causes the star wheel u to rotate to automatically extend the entire length of the upper strut t, thereby maintaining a constant clearance between the brake shoes b, c and the brake drum z.
The drum brake device as described above has need of improvement in the following areas:
Both conventional devices described above have a problem in that the cumulative effect of the tolerances of each component for the parking brake requires a clearance between the idle lever k and one rod l or the other rod m. This generates a partially ineffective stroke or play in the system.
Moreover, as the lining of the other brake shoe c is gradually worn, there is a gradual shift in the point at which the other rod m contacts with the brake shoe c or the idle lever k. That is, as shown in FIG. 10, the amount of displacement .delta. (amount of lining wear) at the brake center of the brake shoe c (pivot point of the brake shoe c and the idle lever k), and the amount of displacement .delta.c and .delta.k of the brake shoe c and idle lever k respectively at the point of the contact with the other rod m are defined as follows: ##EQU1##
In this case, H.sub.3 is considerably smaller than H.sub.1 ; hence, the displacement .delta.k of the idle lever k will be considerably larger than the displacement .delta.c of the brake shoe c. As a result, as the lining wears, the parking brake lever j stroke increases and this increase is disconcerting to the driver. This also may cause the parking brake lever j to interfere with other components, which influences the effectiveness of the brake. Moreover, consideration of the parking brake lever j stroke, the size of the brake shoe, the brake offset, etc., is restricted, which creates a limitation in its design.
Moreover, the respective brake shoes b, c when applying the parking brake are moved so that the adjacent two ends of the brake shoes b, c rotate in the direction to spread apart. In other words, the adjacent ends of the respective brake shoes b, c separate from the anchor block d simultaneously. In this state, if the brake drum starts to rotate together with the wheel, both brake shoes b, c and rods l, m, etc., also rotate together, whereupon one or the other brake shoe collides against the anchor block d. The noise so generated is disconcerting to the driver. Additionally, this impact load is applied repeatedly on the anchor block d; therefore, the strength of the components becomes critical.
In the conventional device as disclosed in Australian Patent Number AU-B1 53 491/79, the cumulative effect of the tolerances of each component will be such that the idle lever k could abut against the other rod or play could be generated. At the very least, play will be generated when the service brake is applied. Accordingly, the idle lever k could vibrate creating a strange noise when the vehicle is in motion or the foot brake is applied. This noise can be disconcerting to the driver.