Reduced noise brake shoe assembly

The present invention is a drum brake shoe assembly having an improved support structure which limits the lateral travel of the leading brake shoe, to thereby reduce or eliminate audible impacts between the brake shoe and the backing plate of the drum brake assembly. In accordance with the invention, two or more spaced apart hold down devices are provided on the leading brake shoe while a single hold down device is used on the trailing brake shoe. Each of the hold down device includes a hold down pin connected at a first end to the backing plate of the drum brake assembly. The second end of each hold down pin extends through an opening in the web of the brake shoe. Springs such as coil springs or the like are disposed between the second ends of the hold down pins and the brake shoe web for urging the brake shoe against the backing plate.

BACKGROUND OF THE INVENTION
 The present invention relates generally to a drum brake shoe assembly and,
 in particular, to a drum brake shoe assembly having an improved brake shoe
 support structure to retain the drum brake shoe in position relative to a
 backing plate.
 Conventional drum brakes include a pair of drum brake shoes each having a
 semicircular rim with a radially outer surface and a band of friction
 material attached thereto. A strengthening web is attached to the rim and
 extends radially inwardly along most of the length of each shoe. The brake
 shoes are mounted on a backing plate of the drum brake assembly and
 supported in place by a hold down mechanism. A conventional hold down
 mechanism consists of a hold down pin having a first end connected to the
 backing plate and a second end extending through a hole in the brake shoe
 web. A coil spring is typically disposed between the second end and the
 web to urge the brake shoe toward the backing plate. While most drum brake
 assemblies use one hold down mechanism per shoe, it is known to use two
 hold down mechanisms for each brake shoe, as disclosed in U.S. Pat. No.
 5,201,390 to Anderson et al. However, using two hold down mechanisms per
 brake shoe is generally undesirable due to the increased cost and
 complexity.
 To brake the vehicle, the brake shoes are moved radially outwards by an
 actuator, such as a hydraulically or pneumatically actuated brake
 cylinder, until the friction material contacts a corresponding friction
 surface on the brake drum. The friction created during contact slows the
 rotation of the drum and wheel, thus slowing the vehicle.
 However, the applicant has discovered that the conventional brake shoe
 support configuration using only one hold down mechanism per shoe allows
 considerable unwanted shoe movement in the lateral direction (also known
 as the axial direction in relation to the brake drum or wheel) during
 braking in response to variations in the surface of the brake drum. The
 applicant has further discovered that the movement of the shoe in the
 lateral direction allows the shoe to impact with the backing plate causing
 unacceptable noise during braking. It is desirable to reduce the braking
 noise by reducing the unwanted shoe movement during braking while keeping
 the added cost and complexity to a minimum.
 SUMMARY OF THE INVENTION
 The present invention is a drum brake shoe assembly having an improved
 support structure which limits the lateral travel of the leading brake
 shoe, to thereby reduce or eliminate audible impacts between the brake
 shoe and the backing plate of the drum brake assembly. In accordance with
 the invention, two or more spaced apart hold down devices are provided on
 the leading brake shoe while a single hold down device is used on the
 trailing brake shoe. Each hold down device includes a hold down pin
 connected at a first end to the backing plate of the drum brake assembly.
 The second end of each hold down pin extends through an opening in the web
 of the brake shoe. Springs such as coil springs or the like are disposed
 between the second ends of the hold down pins and the brake shoe web for
 urging the brake shoe against the backing plate.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
 The present invention relates to a drum brake assembly which is
 conventional and well known to those skilled in the art which includes an
 improved brake shoe support structure. It is to be understood that the
 specific device illustrated in the attached drawings and described in the
 following specification is simply an exemplary embodiment of the invention
 defined in the appended claims. Hence, specific dimensions and other
 physical characteristics relating to the embodiments disclosed herein are
 not to be considered as limiting, unless the claims expressly state
 otherwise.
 Referring now to the drawings, there is illustrated in FIG. 1 a drum brake
 assembly 8 including a backing plate 10 which is fastened to the axle of a
 motor vehicle (not shown). A pair of brake shoes, including a trailing
 brake shoe 12 and a leading brake shoe 13, are fastened to the backing
 plate 10, as described below. Each of the brake shoes 12 and 13 comprises
 a rim 14 bent in circular arc shape to support a friction lining 16 which
 has been secured to the rim 14. A brake shoe web 18 is secured to the rim
 14 and is arranged so as to be in a parallel orientation to the backing
 plate 10. As is well known, a thrust bearing 20 is secured to the backing
 plate 10 between the two brake shoes 12 and 13, and a wheel brake cylinder
 22 is disposed diametrically opposite the thrust bearing 20. The drum
 brake assembly 8 is preferably a hydraulic braking system having a
 hydraulically actuated wheel brake cylinder 22, alternatively, the drum
 brake assembly may be pneumatically actuated.
 Each of the two brake shoes 12 and 13 is held in engagement with the
 backing plate 10 by a centrally located primary hold down device 30. In
 addition, in accordance with the present invention, the leading shoe 13 is
 further provided with at least one additional, secondary hold down device
 31 which is spaced apart from the primary hold down device 30. Each of the
 hold down devices 30 and 31 employed in accordance with the invention may
 be of identical structure, and therefore only the hold down device 31 will
 be shown and described in greater detail.
 As best illustrated in FIG. 2, the hold down device 31 includes a hold down
 pin 32 which passes through a hole 34 provided in the backing plate 10 and
 through a hole 36 in the web 18 of the leading brake show 13. One end of
 the hold down pin 32 is provided with a head 38 which is adapted to abut
 the adjacent surface of the backing plate 10. The opposite end of the hold
 down pin 32 is provided with a second head 40 which, when the first head
 38 is in abutment with the backing plate 10, is spaced apart from the web
 18 of the brake shoe 13.
 The hold down device 31 further includes a spring assembly 42 extending
 between the second head 40 of the hold down pin 32 and the web 18 of the
 brake shoe 13. In cooperation with the hold down pin 32, the spring
 assembly 42 urges the brake shoe 13 towards the backing plate 10 to retain
 the brake shoe against the backing plate thereby inhibiting lateral
 movement of the shoe. The spring assembly 42 may be comprised of any
 suitable spring or spring-like device, such as a leaf spring or the
 helical spring illustrated in the drawings.
 During braking, the vehicle operator depresses the brake pedal (not shown)
 which hydraulically actuates the wheel cylinder 22. The brake shoes 12 and
 13 are displaced in the radially outward direction by the wheel brake
 cylinder 22 pivoting against the thrust bearing 20. The friction linings
 16 are applied against the friction surface of the brake drum (not shown)
 in the conventional manner to brake the vehicle. As per industry
 convention, the friction lining of the leading brake shoe 13 contacts the
 friction surface of the brake drum before the trailing brake shoe friction
 lining, hence the derivation of the terms leading and trailing brake
 shoes.
 As described above, the prior art drum brake assembly preferably uses a
 single hold down device for each brake shoe to retain the shoes against
 the backing plate. Using a single hold down device per shoe has typically
 been preferred over other known assemblies using two hold down devices for
 each shoe because of the greater simplicity and lower costs of using fewer
 hold down devices. However, it was found that while the conventional
 single hold down device 30 does adequately prevent lateral movement of the
 trailing brake shoe 12, the leading shoe 13 is not adequately retained
 against the backing plate 10. With a single hold down device 30, the
 leading shoe 13 was found to have an undesirable amount of lateral
 movement during braking. As a result, the leading shoe 13 was free to
 periodically impact against the backing plate 10, causing an unacceptable
 level of noise from the drum brake assembly 8.
 The movement of the leading brake shoe 13 during operation is complex, but
 has been found to occur primarily in lateral planes in response to
 variations in the surface of the brake drum as installed in a motor
 vehicle. Outboard tight spots between the friction lining 16 and the drum
 friction surface cause the brake shoe 12 to move laterally away from the
 backing plate 10, while inboard tight spots cause the brake shoe to move
 laterally towards the backing plate. The result is an often audible impact
 between the leading brake shoe 13 and the backing plate 10 for each
 rotation of the associated motor vehicle wheel.
 Accordingly, it was found that using two, spaced apart hold down devices
 30, 31 on the leading shoe 13, the lateral travel of the shoe 12 is
 greatly restricted as compared with the single point shoe support
 structure conventionally employed. The present invention thus reduces or
 even eliminates the occurrence of audible impact between the brake shoe
 and the backing plate of the drum brake assembly during the operation of
 the associated motor vehicle.
 As noted above, while two spaced apart hold devices on the leading shoe are
 sufficient to provide significantly improved noise reduction of the drum
 brake assembly, more than two such hold down devices may be employed to
 further restrict lateral movement of the leading brake shoe relative to
 the backing plate. In addition, in a preferred embodiment, the two or more
 hold devices are spaced as far apart as is practicable to maximize the
 restriction in the lateral movement of the brake shoe.
 In accordance with the provisions of the patent statutes, the present
 invention has been described in what is considered to represent its
 preferred embodiment. However, it should be noted that the invention can
 be practiced otherwise than as specifically illustrated and described
 without departing from its spirit or scope.