Abstract:
A slip clutch acts between a shaft and a housing. The slip clutch includes an oil reservoir within the housing and one or more slipping surfaces disposed in the oil reservoir that are keyed to the shaft and possibly alternately to the housing. The slipping surfaces may include stacked plates or washers. Each of the washers includes an oil groove or other structure on one surface thereof that maintains a consistent film of oil on the washer surfaces. The stacked washers slip relative to one another such that the shaft rotates relative to the housing upon an application of torque that exceeds a predetermined torque.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     (Not Applicable) 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     (Not Applicable) 
     BACKGROUND OF THE INVENTION 
     The invention relates to a slip clutch and, more particularly, to a slip clutch including components having at least one slipping surface that results in a lightweight and consistent overload clutch assembly. 
     There exist known structures to act as an overload clutch between two members, such as a rotating shaft and a housing. A clutch typically serves to prevent an excessive load on one part or the other by permitting the shaft and housing to slip relative to each other upon the application of excessive torque. 
     In most applications, it is important that activation of the clutch occurs consistently. Moreover, the clutch must be robust to endure normal use. One known clutch arrangement utilizes a slipping plate or the like set on ball bearings. During regular use, however, the ball bearings tend to fracture, eventually requiring replacement of the clutch. Moreover, conventional designs are typically formed of heavier materials, which can be a concern if total weight of the device in which the clutch is installed is an important design consideration. 
     In activating drive functions in industrial vehicles and the like, ANSI (American National Standards Institute) standards require two independent actions. For example, in an aerial work platform, a user may be required to both press and hold an activation switch or the like and operate a controller to effect movement of the platform. 
     Higher end hand-held power drills are capable of producing sufficient torque to provide drive power for various industrial vehicle functions including for example drive, lift, etc. Additionally, the battery life of drills is expanding with advancing battery technology. Power drills, however do not satisfy ANSI standards, since only a single action of activating the trigger is required to power the drill. 
     BRIEF SUMMARY OF THE INVENTION 
     The slip clutch described herein utilizes lightweight materials of a simple construction that provides consistent and accurate activation upon the application of a torque exceeding a predetermined torque. Although the slip clutch described herein has broad applications, an exemplary application is for use with a mast lifting apparatus including an operator platform that can be raised and lowered on a mast. In the exemplary application, the shaft and housing are rotated by a source of motive power such as a hand-held power drill or the like to drive a worm gear assembly, which in turn rotates a drum to lift the platform on the mast. If the platform reaches its topmost position or becomes stuck in any manner, the application of torque by the source of motive power will exceed an allowable limit, which will thereby activate the clutch described herein to prevent damage to the lifting components. 
     In order to utilize a hand-held power drill as a source of motive power, it is necessary to satisfy ANSI requirements with regard to two independent actions. The shaft cooperable with the clutch assembly described herein includes a fitting for receiving a drill bit, and the shaft and clutch assembly are configured such that two independent actions are required to activate the drive system. For example, a first action may be to press/hold down the shaft, and a second action may be to activate the drill. 
     In an exemplary embodiment of the invention, a slip clutch acts between a shaft and a housing. The slip clutch includes an oil reservoir within the housing and a plurality of stacked washers disposed in the oil reservoir and being alternately keyed to the shaft and the housing. Each of the washers includes an oil groove or a plurality of oil grooves on one surface thereof, where the oil grooves maintain a consistent film of oil on the washer surfaces. The stacked washers slip relative to one another such that the shaft rotates relative to the housing upon an application of a predetermined torque. The slip clutch may additionally include a compression spring disposed within the housing that acts to maintain a minimum compression force on the stacked washers. A cap member may be secured in the housing, which includes a central opening therein receiving the shaft. The compression spring is preferably disposed surrounding the shaft between the cap member and the stacked washers. The cap member may be threaded in the housing, where the predetermined torque is adjustable according to a position of the cap member relative to the housing. Alternatively or additionally, the predetermined torque is adjustable according to a number of washers in the stacked washers. 
     In another exemplary embodiment of the invention, a slip clutch includes a first plurality of washers connected to the shaft and a second plurality of washers connected to the housing. The first and second pluralities of washers are immersed in an oil bath within the housing, and the washers comprise structure that maintains a consistent film of oil thereon. 
     In yet another exemplary embodiment of the invention, a slip clutch includes at least one slipping surface disposed against a surface of the housing and keyed to the shaft, and a biasing mechanism urging the slipping surface into contact with the housing surface. The slipping surface is immersed in an oil bath within the housing. 
     In still another exemplary embodiment of the invention, a method for preventing a torque overload with a slip clutch acting between a shaft and a housing includes the steps of stacking a first plurality of washers connected to the shaft together with a second plurality of washers connected to the housing; immersing the first and second pluralities of washers in an oil bath; and maintaining a consistent film of oil on washer surfaces, wherein the stacked washers slip relative to one another such that the shaft rotates relative to the housing upon an application of a predetermined torque. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other aspects and advantages of the present invention will be described in detail with reference to the accompanying drawings, in which: 
         FIG. 1  is a perspective view of the clutch assembly described herein; 
         FIG. 2  is a cross-sectional view of the clutch assembly; 
         FIG. 3  is an exploded view showing an exemplary clutch plate or washer stack; 
         FIG. 4  illustrates alternative clutch plate designs; 
         FIG. 5  shows an actuation feature of the clutch assembly in a disengaged position; and 
         FIG. 6  shows the actuation feature in the engaged position. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  is a perspective view of an overload clutch assembly  10 . The clutch assembly  10  acts between a shaft  12  and a housing  14 . An engagement gear  16  or the like is rotatable with the housing  14 . Also shown is a mounting plate  18  for suitably mounting the assembly. 
       FIG. 2  is a cross-sectional view of the clutch assembly  10  through the housing  14 . The clutch assembly  10  preferably includes a plurality of stacked clutch plates or washers  20  through which the shaft  12  is inserted.  FIG. 3  is an exploded view of the stacked washers  20 . The washers  20  include preferably alternating small plates  22  and large plates  24 . The small plates  22  are keyed to the shaft  12  by a suitably shaped opening  26  through which the shaft  12  is inserted. The shaft  12  includes a bottom section  12   a  including key surfaces that engage the corresponding surfaces in opening  26 . In this manner, the small plates  22  are rotated with rotation of the shaft  12 . 
     The large plates  24  are keyed to the housing  14  by exterior key surfaces  28 . The key surfaces  28  engage correspondingly shaped surfaces  14   a  within the housing  14 . 
     With continued reference to  FIG. 2 , a cap member  30  is secured in the housing  14  preferably via a threaded connection and together with an O-ring  32  delimits an oil reservoir  34  within the housing  14 . The housing  14  is closed at its bottom and thus contains oil in the oil reservoir  34 . The cap member  30  includes a central opening  36  therein receiving a shaft  12 . A compression spring  37  is disposed over the shaft  12  between the cap member  30  and a top plate  38  engaging the washer stack  20 . A top plate  38  serves to distribute the spring force of the compression spring  37 . The spring  37  acts on one regard to maintain a minimum compression force on the stacked washers  20 . 
     As shown in  FIG. 3 , each of the small plate washers  22  and the large plate washers  24  includes one or more oil grooves  40  therein. Alternative configurations for the washer plates  22 ,  24  are shown in  FIG. 4 , referenced as  22 ′,  24 ′. The oil grooves  40  serve to maintain a consistent film of oil  41  on the washer surfaces by permitting oil in the reservoir to flow between the washers and thereby access the washer surfaces. Of course, those of ordinary skill in the art will appreciate alternative designs and configurations for the washer plates, and the invention is not necessarily meant to be limited to the described and illustrated structure. For example, any other arrangement may be utilized to permit oil from the oil reservoir  34  to flow onto the washer surfaces such as via through holes, slots, notches, etc. 
     In use, as the shaft  12  is rotated, the housing  14  rotates, which in turn rotates the engagement gear  16 . Upon the application of a torque exceeding a predetermined torque, the small plate washers  22  keyed to the shaft  12  will slip relative to the large plate washers  24  keyed to the housing  14 . As such, the shaft  12  will continue to rotate while the housing  14  remains stationary. 
     A preferred material for the washers  22 ,  24  is hardened steel, which provides excellent consistency and robustness. The collection of materials is very lightweight compared to conventional clutch arrangement. Moreover, by immersing the washer stack  20  in an oil reservoir  34 , the predetermined torque over which the clutch assembly  10  is activated is accurate and consistent regardless of environmental conditions, such as temperature, humidity, etc. as the characteristics of the oil do not vary significantly in different environmental conditions or over time. The consistency is provided by the metal against metal oil film engagement, which is not attainable with a rubber brake pad or the like against a metal rotor. Moreover, the performance of brake pads and similar rubber or synthetic products could vary considerably in different environmental conditions. 
     The specific predetermined torque can be set for a specific application and can be adjusted by adjusting a spring force of the spring  37 , by changing the number of washers in the washer stack  20 , or by adjusting a position of the cap member  30 , e.g., via the threaded connection to the housing  14 . A lower compression force by spring  37  reduces the predetermined torque, and a higher spring force increases the predetermined torque. 
     With the structure configured as described herein, a torque range for activation/deactivation of the clutch can be 15% or less. That is, assuming it is desirable for the shaft  12  to slip upon an application of 2.5 ft-lbs or higher, the structure described herein is consistent and accurate to ensure that a driving torque of 2.2 ft-lbs or lower will never activate the clutch, regardless of environmental conditions. This consistent but small window is beneficial in designing components in which the clutch assembly is incorporated. 
     Although the clutch assembly described herein is shown using a plurality or stack of washers  20 , the assembly may function properly with as little as a single plate member against a bottom wall of the housing  14 . The number of plate members/washers may be varied to vary the torque activation level and/or the weight of the assembly. 
     A second component of the clutch assembly will be described with reference to  FIGS. 5 and 6 . A stop plate  42  is coupled with a lower end of the housing  14  (see  FIG. 2 ). The mounting bracket  18  includes a fixed member  44  to which the rotating plate  42  is selectively engageable. In an idle position (as shown in  FIG. 6 ), a spring force  46  acting below the engagement gear  16  urges the shaft toward a retracted position where the rotating plate  42  engages the fixed member  44 , thereby preventing rotation of the shaft  12 . In order to free the rotating plate  42  from the fixed member  44 , the operator is required to press down on the shaft  12  against the spring force  46  to clear the rotating plate  42  from the fixed member  44 . 
     In the exemplary application to a mast lift described above, the shaft  12  may include an operating end  48  ( FIG. 1 ) shaped to engage a motive power source. An exemplary motive power source is a hand-held power drill. In use, the power drill can be provided with a suitably-sized bit for engaging the shaft end  48 . The user engages the power drill with the shaft end  48  and presses down on the shaft  12  to displace the shaft  12  from its retracted position ( FIG. 6 ) to its extended position ( FIG. 5 ). Subsequently, the user can activate the drill to rotate the shaft  12 . Activation of the shaft  12  thus requires two independent actions, being press down and pull trigger. It is preferable that the two actions are performed simultaneously. It is further preferable that the shaft  12  be first pressed down as pulling the trigger without pressing down on the shaft  12  would activate the slip clutch assembly described above. In this manner, this provides a safety feature in the event the drill or other power source slips off the shaft  12  during use. Moreover, the use of two independent actions satisfies ANSI standard requirements. 
     An alternative arrangement in the exemplary mast lift application utilizes a power pack that is securable over the shaft. The power pack includes an internal source of motive power to rotate the shaft  12  according to an activation button or the like on the power pack. When the power pack is attached to the unit, the power pack displaces the shaft  12  to the extended position. The power pack itself may include a two-action activator such as a lever with a deadman switch or the like. 
     The clutch assembly described herein utilizes lightweight and inexpensive parts while performing accurately and consistently. The structure is robust and can withstand normal use over longer periods of time than with conventional assemblies. The clutch assembly enables efficient use of a hand-held power drill or the like to safely and efficiently provide a rotating force for a gear assembly while preventing damage due to overloading. 
     While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiments, it is to be understood that the invention is not to be limited to the disclosed embodiments, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.