The present invention relates to spring-type brake actuators, and in particular to arrangements of power springs within such actuators to provide increased parking brake actuation force.
So-called “spring brake” actuators are commonly used to provide service, parking and emergency brake operation on vehicles such as commercial trucks, tractors and trailers equipped with lever-operated drum or disc brakes. Spring-type brake actuators are typically pneumatically operated, and are supplied with operating air from a compressed air source on the vehicle. These actuators also typically are arranged in a “fail-safe” manner, i.e., where the actuator defaults to a brake application state upon loss of operating air pressure.
An example prior art spring brake actuator is shown in cross-section view in
When no pneumatic pressure is present in the
The vehicle brake is applied as a service brake during normal operation by admitting compressed air into the front ventilation chamber 7 (via a port not shown in
As discussed in pending U.S. patent application Ser. No. 11/012,313, filed Dec. 16, 2004, prior art spring-type brake actuators have a number of problems. application Ser. No. 11/012,313 discloses an improved actuator which is safer, lighter, simpler, more reliable, less costly and/or safer to assemble and service than prior art actuators. As shown in
The parking brake release actuator, instead of pressing directly on the service brake actuator (as in the prior art), is affixed via its attached shaft 200 to the intermediate spring plate 160. Thus, when air pressure is applied to the rear chamber, rather than compressing the brake actuator spring into the rear end of the actuator housing, as in the prior art, the parking brake release actuator draws the intermediate spring plate toward the intermediate body portion 110 of the actuator, compressing the brake actuator spring against the front side (or “floor”) of the intermediate flange to remove the spring's force from the actuator rod. This arrangement preserves the “fail-safe” nature of the spring-type brake actuator (i.e., loss of pressure in the rear chamber still results in the brake actuator spring re-applying the brake), while also positively capturing the spring between the spring plate and the intermediate flange.
One of the features of the new spring brake actuator is the location of the brake actuator spring 140 immediately adjacent to the front chamber 300, where it can generate substantial force to actuate the brake in a “parking brake” mode when pressure is released from parking brake chamber 230. The amount of force the power spring generates is determined by a number of factors, including the spring material, the diameter of the spring wire, the diameter of the spring, the spring length, the spring's coil pitch, and the distance the spring is displaced from its unloaded length.
As a general rule, the greater the desired parking brake actuation force, the larger the spring must be (e.g., larger coil wire, spring diameter, and/or length). One approach to obtaining greater parking brake actuation force would be to enlarge the diameter power spring. However, enlarging the spring would require that the actuator housing also be enlarged to accommodate the larger spring. Enlargement of the brake actuator housing may be undesirable for a number of reasons, including the need to minimize actuator size in order to fit within limited space envelopes in commercial vehicle brake applications, and the need to incur substantial additional costs for designing, manufacturing and supporting multiple sizes of spring brake actuator housings. These latter concerns become particularly acute when larger housings must be provided, but because demand for the larger housings would likely be limited, the larger spring brake actuators would be unprofitable at market-acceptable prices.
Alternatively, if the diameter and/or length of the spring cannot be increased, the spring rate (the amount of force required to displace the spring over a given distance) generally must be increased by increasing the diameter of the coil wire used in the spring and/or by using a stiffer (i.e., lower elasticity) material for the coil. However, achieving greater parking brake actuation force by simply increasing spring rate is not a preferred approach, at least in part due to concerns with increased component cost and potentially lower fatigue life of larger, stiffer spring materials.
In view of the foregoing, it is an objective of the present invention to provide an improved power spring arrangement in which different parking brake actuation force levels may be provided within a single actuator housing design.
It is a further objective of the present invention to provide a power spring arrangement which eliminates the need to design, tool, manufacture and support different size brake actuator housings in order to meet higher parking brake actuator force demands.
In addressing these and other objectives, the present invention includes a plurality of concentric power springs located in the power spring cavity of the actuator housing. The springs are preferably provided with a separator between adjacent springs, and the separator is preferably provided with flanges at its ends to receive opposite ends of the adjacent concentric spring coils. In addition to precluding interference between the concentric springs' coils, such a separator causes adjacent springs to act in “series,” i.e., generates a reaction force which passes in series from a rear of the power spring cavity through a first spring, the separator, and a second spring to the spring retainer plate.
This series application effectively extends the distance over which the combined concentric springs can apply a high parking brake actuation force. The separator allows the springs to be nested one within the other when the springs are fully compressed into the actuator's power spring cavity, while simultaneously allowing one spring to effectively serve as the seat for its adjacent spring. Thus, when the power springs are allowed to advance in a brake application direction, the displacement of the separator towards the brake by the one spring advances the adjacent concentric spring's seat (the separator flange) toward the brake. As a result, the adjacent spring is also displaced toward the brake, allowing it to exert its spring force over a greater distance than if it were seated against the rear of the power spring cavity. Accordingly, at any given distance from the power spring cavity, the brake application force applied to the spring retainer by the series concentric springs will be higher than if both springs were resting against the rear of the power cavity.
The concentric spring arrangement allows a single brake actuator housing to accommodate both a “standard” single power spring which provides sufficient parking brake actuation force for most applications, and to accommodate multiple power springs to provide a higher parking brake actuation force in more demanding brake applications. The use of multiple concentric power springs also allows the individual springs' spring rates to remain in a desirably low range, while providing a combined, overall spring rate that generates the desired parking brake actuation force within the spring length limits of the single brake actuator housing.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.
As illustrated in
As shown in
It will be apparent to one of ordinary skill in the art that the relative strengths of the individual concentric springs to one another may be varied while retaining the functionality of the foregoing embodiment and remaining within the scope of the present invention. One of ordinary skill will also recognize that the spring parameters (e.g., coil diameter, wire diameter, wire material, free length), also may be altered as necessary to ensure that the combined force of the plurality of power springs is sufficient to generate the desired parking brake actuation force at the spring plate throughout the range of motion of the brake actuator rod 34.
The use of multiple concentric power springs, and in particular concentric springs in a series arrangement such as that shown in the
The foregoing disclosure has been set forth merely to illustrate the invention and is not intended to be limiting. For example, while the springs illustrated herein are formed from coil-wound wire, one of ordinary skill would recognize that other spring configurations may be readily substituted. For example coils of flat wire or other non-coil spring configurations may be employed; alternatively, an array of spring elements may be considered, such as a plurality of small-diameter springs arranged at close centers in two concentric large-diameter circles may be provided in place of two large individual concentric coil springs Similarly, the spring separator need not be a one-piece cup-shaped separator, but for example, may comprise a plurality of metal strips spaced about the annulus between two concentric springs. Because other such modifications of the disclosed embodiments incorporating the spirit and substance of the invention may occur to persons skilled in the art, the invention should be construed to include everything within the scope of the appended claims and equivalents thereof.