Abstract:
A brake of a concrete finishing trowel responds automatically to the cessation of drive torque delivery to the rotor to actively brake the trowel&#39;s drive train, hence inhibiting or preventing frame rotation. The brake preferably takes advantages of inherent characteristics of an inclined gear, such as a worm of a gearbox, to permit an internal component of the gearbox to shift automatically upon the cessation of drive torque therethrough from a brake released position to a brake engaged position. In the case of a worm gear-based gearbox, the worm is configured such that reaction forces that are normally generated by the delivery of drive torque therethrough shift the worm to release the brake during normal trowel operation. These forces are eliminated in the absence of drive torque transfer, permitting the worm to shift to a brake engaged position.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to concrete finishing trowels and, more particularly, relates to a concrete finishing trowel having a drive train that is braked automatically upon the cessation of drive torque delivery therethrough. The invention additionally relates to an automatically braked gearbox usable with such a trowel and to a method of braking such a gearbox. 
     2. Discussion of the Related Art 
     Walk behind trowels are generally known for the finishing of concrete surfaces. A walk behind trowel generally includes a rotor formed from a plurality of trowel blades that rest on the ground. The rotor is driven by a motor mounted on a frame or “cage” that overlies the rotor. The trowel is controlled by an operator via a handle extending several feet from the cage. The rotating trowel blades provide a very effective machine for finishing mid-size and large concrete slabs. However, walk behind trowels have several drawbacks. 
     For instance, the rotating blades impose substantial torque on the cage that normally is counteracted by the operator through the handle. If the operator releases the handle while the rotor is being driven, the torque may cause the trowel to spin undesirably. In order to avoid this potential problem, some trowels employ an automatic disconnect or kill switch that shuts down the engine or otherwise stops delivery of drive  torque to the rotor if the operator releases the handle and/or if a sensor determines that the trowel is spinning at a rate indicative of loss of operator control. A trowel employing an automatic engine shut-down mechanism is disclosed, for example, in U.S. Pat. No. 2,734,932 to Barnes. A trowel that relies on release of a dead man lever or similar structure to shut down a trowel&#39;s engine or disengage a clutch upon operator release of a steering handle is disclosed in U.S. Pat. No. 4,629,359 to Sengupta. 
     However, merely shutting down the engine or otherwise ceasing the delivery of drive torque to the trowel&#39;s rotor does not necessarily prevent the trowel from spinning because the handle and cage have considerable momentum at the time of shut down. The trowel may spin through a substantial arcuate range of a complete revolution or more while that momentum is being spent. This slow response to an attempted shut down is undesirable. 
     Attempts have been made to mitigate this problem through the incorporation of active brakes in the trowel&#39;s drive train that are designed to prevent or at least inhibit rotation of the trowel&#39;s frame. 
     For instance, U.S. Pat. No. 4,280,980, assigned to Stone, discloses a combined clutch/brake coupling a drive belt of the trowel&#39;s drive system to the input shaft of the trowel&#39;s gearbox. The clutch must be manually-engaged by displacing a lever mounted on the handle. The clutch may be manually disengaged by manual operation of the handle. It may also be disengaged automatically through the operation of a centrifugal weight that is responsive to undesirable rapid spinning of the trowel. In either event, movement of the handle to the clutch disengaged position activates a spring-applied pad-type brake that acts on the drive belt. Although this device works reasonably well, it  exhibits several drawbacks. For instance, it is usable only with a manually actuated clutch that is actuated by the operator only at the expense of release or partial release of the handle. The clutch activation lever also is open to the outside, exposing the internal components of the clutch to contamination by dirt, debris, water, etc. In addition, the clutch brake engages automatically only if a mechanical sensor indicates that the trowel is undergoing unacceptable centrifugal forces. It does not react to an “ordinary” engine shut down situation in which torque transfer to the rotor ceases in the absence of such centrifugal forces. The brake also imposes significant drag on the drive belt, accelerating wear on the drive belt. Finally, braking forces imposed on the clutch are generated solely by the strength of the spring and, therefore, are independent of backdrive forces imposed on the system&#39;s drive train by the rotor. 
     Other brakes are available for other applications that brake a clutch directly rather than braking a drive belt leading from the clutch to the gearbox. Known clutch brakes lack some of the drawbacks of the Stone clutch brake but have drawbacks of their own, rendering them poorly suited for use with a trowel. One such clutch brake is manufactured by North American Clutch Manufacturing (NORAM). The NORAM brake is a manually engaged brake taking the form of a deadman&#39;s switch operated by a control lever much like that commonly found on a walk behind lawnmower. The brake is engaged automatically when the operator releases the control lever to brake the clutch. This brake also works reasonably well, but requires that the operator squeeze the control lever at all times while operating the trowel. This continuous squeezing requirement leads to considerable operator fatigue and also requires that the operator divert a substantial portion of his or her attention to operation of the control lever, hindering his  or her ability to adequately steer the trowel. In addition, the NORAM clutch brake, like that employed by the brake of the Stone machine, imposes braking forces that are independent of the magnitude of backdrive forces generated by the machine&#39;s output. In addition, as with the Stone system, the range of clutches useful with this type of system is considerably limiting. 
     Still another type of clutch brake, offered by Ogura, is electrically powered. An electrically powered clutch brake requires the presence of a reliable power supply to prevent the brake from wearing or seizing. The magneto employed as an electrical power source for a typical walk behind trowel is ineffective for the purpose. Adding additional components to the trowel to upgrade the power supply would add additional cost and weight to the trowel. 
     The need therefore has arisen to automatically brake a drive train of a concrete finishing trowel upon the cessation of drive torque delivery to the trowel&#39;s rotor without interfering with the operator&#39;s ability to steer and control the trowel and without significantly adding to the cost or complexity of the trowel. 
     The need has also arisen to provide a drive train brake that satisfies the precedingly described need and that does not significantly add to the cost or weight of the trowel. 
     The need has additionally arisen to provide a brake that satisfies the first-mentioned need while still being compatible with a variety of different drive systems.  
     SUMMARY OF THE INVENTION 
     Pursuant to the invention, a brake is incorporated into a concrete finishing trowel&#39;s drive train that responds automatically to the cessation of drive torque delivery to the rotor to actively brake the drive train, hence inhibiting or preventing rotation of the trowel&#39;s frame. In a preferred embodiment, the brake takes advantages of inherent characteristics of an inclined gear of a gearbox to permit an internal component of the gearbox to shift axially upon the cessation of drive torque delivery to the gearbox to engage the brake. For instance, in the case of a worm gear-based gearbox, the brake may be mounted on a worm shaft or related component of the gearbox that is responsive to reaction forces imposed thereon upon the delivery of drive torque thereto. The component shifts axially in one direction to release the brake in the presence of the reaction forces and shifts axially in an opposite direction in the absence of the reaction forces to apply the brake. The resulting system has several advantages over clutch-type brakes used on other systems. It is also compatible with any clutch and any torque delivery system. In addition, because it is engaged and released fully automatically, it does not in anyway interfere with or hinder the operator&#39;s ability to steer or otherwise operate the machine. 
     These and other advantages and features of the invention will become apparent to those skilled in the art from the detailed description and the accompanying drawings. It should be understood, however, that the detailed description and accompanying drawings, while indicating preferred embodiments of the present invention, are given by way of illustration and not of limitation. Many changes and modifications may be made  within the scope of the present invention without departing from the spirit thereof, and the invention includes all such modifications. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A preferred exemplary embodiment of the invention is illustrated in the accompanying drawings in which like reference numerals represent like parts throughout, and in which: 
         FIG. 1  is a perspective view of a walk-behind trowel incorporating an automatic gearbox brake constructed in accordance with a preferred embodiment of the present invention; 
         FIG. 2  is a sectional side elevation view of a gearbox of the trowel of  FIG. 1 ; 
         FIG. 3  is a sectional top plan view of the gearbox of  FIG. 2 , showing a gearbox brake in a disengaged position thereof; 
         FIG. 4  corresponds to  FIG. 3  and shows the brake in an engaged position thereof; and 
         FIG. 5  is an exploded perspective view of the brake of  FIGS. 3 and 4 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     1. Resume 
     As indicated above, the invention resides in the automatic braking of an active component of a rotary trowel&#39;s drive train in response to the cessation of torque transfer to the rotor from the trowel&#39;s prime mover in order to inhibit or prevent undesired rotation of the trowel&#39;s frame and handle. Preferably, the brake moves with a component  of the drive train that is subject to axial reaction forces upon the delivery of drive torque therethrough. A preferred embodiment of the invention will now be described in conjunction with a walk behind trowel having an internal combustion engine as a prime mover and a brake built into the trowel&#39;s gearbox. However, the invention is not so limited. It also potentially applies to ride on trowels and/or to a brake that acts on components of the trowel&#39;s drive system other than the gearbox. 
     2. System Overview 
     Referring to  FIG. 1 , a walk behind trowel  10  is illustrated that incorporates a gearbox brake assembly  100  ( FIGS. 3–5 ) constructed in accordance with a preferred embodiment of the invention. In general, the walk behind trowel includes a rotor  12 , a frame or “cage”  14  that overlies and is supported on the rotor  12 , an engine  16  that is supported on the frame  14 , a drive train  18  operatively coupling the engine  16  to the rotor  12 , and a handle  20  for controlling and steering the trowel  10 . Referring to  FIGS. 1 and 2 , the rotor  12  includes a plurality of trowel blades  22  extending radially from a hub  24  which, in turn, is driven by a vertical shaft  26 . The shaft  26  of this embodiment comprises the gearbox output shaft (detailed below). Alternatively, the shaft  26  could be coupled to the gearbox output shaft either directly or via an interfering torque transfer arrangement. 
     The motor  16  comprises an internal combustion engine mounted on the cage  14  immediately above the rotor  12 . Referring again to  FIG. 1 , the engine  16  is of the type commonly used on walk behind trowels. It therefore includes a crankcase  30 , a fuel tank  32 , an air supply system  34 , an output shaft (not shown), etc. The motor  16  further  includes an ignition system that includes a magneto ignition rotor (not shown). The magneto ignition rotor rotates with the engine output shaft and generates an electrical pulse with every cycle that is sufficiently strong to power a spark plug but insufficiently strong to power electronically actuated peripheral devices such as valves and electronic clutch brakes. 
     The drive train  18  may be any structure configured to transfer drive torque from the engine output shaft to the rotor input shaft  26 . In the illustrated embodiment, it comprises a centrifugal clutch (not shown) coupled to the motor output shaft and a gearbox  40  that transfers torque from the clutch to the rotor input shaft  26 . Referring to  FIGS. 1–5 , the illustrated gearbox  40  is a worm gearbox of the type commonly used on walk behind trowels. It includes a housing  42 , an input shaft  44  extending horizontally from the housing  42 , the above-described shaft  26  extending vertically from the bottom of the housing  42 , and an internal torque transfer system  46  delivering torque from the input shaft  44  to the output shaft  26  and effecting any desired speed change ratio. The input shaft  44  is coupled to an output shaft  48  for the clutch, either directly or, more preferably, via an intervening drive such as a belt drive that includes a drive pulley  50 , a belt  52 , and a driven pulley  54  (all best seen in  FIG. 1 ). A gear drive or chain drive would also suffice for this purpose. 
     Referring to  FIGS. 2–5 , the housing  42  comprises a metal casting that is bolted to the cage  14  and that is connected to the handle  20  by a bracket  55 . The housing  42  includes inner and outer (left and right) walls  56  and  58 , side walls  59  and  60 , and upper and lower walls  61  and  62 . Side and top covers  64  and  66  shield openings in the outer and upper walls  58  and  61 , respectively, and are selectively removable to expose interior  components of the gearbox  40  for inspection or replacement. The input shaft  44  extends horizontally into the housing  42  from an outer end thereof located outside of the housing  42  to an inner end located within the inner wall  56 . The input shaft  44  is borne by the housing  42  by a first bearing  68  located in the side cover  64  and a second bearing  70  located in a recess bored into the inner wall  56 . As described in more detail below, the bearings  68  and  70  are configured to permit limited sliding movement of the input shaft  44  relative to the housing  42 . The output shaft  26  extends vertically into the housing  42  from a first end located below the housing to a second end located above the input shaft  44 . The output shaft  26  is rotatably borne in the housing  42  by an upper bearing  72  borne by the upper cover  66  and a lower bearing  74  borne by a boss  76  formed in the bottom wall  62  of the housing  42 . 
     Referring now to  FIGS. 2–5 , the torque transfer system  46  of this embodiment includes a worm drive that is configured to convert the horizontal rotation of the input shaft  44  to the vertical rotation of the output shaft  26  while also effecting any desired speed change. The worm drive  46  includes a worm  80  and a worm gear  82 . The worm gear  82  is keyed or otherwise affixed to the output shaft  26  between the upper and lower bearings  72  and  74  and meshes with the worm  80  such that rotation of the worm  80  about a horizontal axis drives the worm gear  82  and the output shaft  26  to rotate about a vertical axis. The worm  80  is disposed on and preferably formed integrally with the outer periphery of the input shaft  44 . As a result, the shaft  44  can be conceptually divided into an input section and a worm section. Alternatively, a worm could be formed from a gear mounted on the input shaft  44  or on another shaft that mates directly or indirectly with the input shaft.  
     3. Construction and Operation of Brake 
     As mentioned above, the gearbox  40  is provided with a brake  100  that releases automatically in response to reaction forces on the worm  80  and that is otherwise engaged. More specifically, like all worms, the threads of the worm  80  are “inclined” to the extent that they extend at an angle relative to radial bisectors of the threads. The transfer of torque to the worm gear  82  from these inclined teeth imposes reaction forces that tend to drive the worm  80  and the entrained input shaft  44  axially away from the worm gear  82  or to the left in  FIGS. 3 and 4 . Most gearboxes are designed to prevent such axial motion. However, pursuant to an embodiment the invention, measures are taken to permit these reaction forces to drive the worm  80  and input shaft  44  axially relative to the housing  42  to release the brake  100 . This effect is achieved by configuring the input shaft  44  and/or bearings  68 ,  70  to be movable axially relative to the housing  42 . In the illustrated embodiment, the input shaft  44  is configured to be moveable axially relative to the inner bearing  70 , and the outer bearing  68  and input shaft  44  are configured to be movable axially relative to the housing  42 . This combination is employed because the inner bearing  70  is relatively lightly loaded, permitting a relatively small needle bearing to be used. Needle bearings can accommodate limited axial movement between themselves and the supported shafts. Conversely, the outer bearing  68  takes up most of the load and, therefore, preferably comprises a tapered roller bearing. Tapered roller bearings cannot accommodate movement between themselves and the borne shaft but can be configured to move axially with the shaft, as is the case in the present embodiment. As a result of this configuration, the worm  80 , shaft  44 , and bearing  68  are driven axially or to the left in  FIGS. 3 and 4  upon the transmission of drive  torque to the worm gear  82 . The stroke of this movement is relatively small—on the order of 1 to 2 mm, but more than sufficient to release the brake  100 . 
     The brake  100  is configured to be automatically responsive to input shaft motion to disengage when the input shaft  44  shifts to the left upon the transmission of drive torque therethrough, and to automatically engage when the input shaft  44  moves in the opposite direction, either under backdrive forces imposed thereto by the worm gear  82  or by a return spring (detailed below). A variety of different brakes cooperating with the input shaft directly or indirectly in a variety of different manners could operate in this manner. In the currently preferred embodiment, the brake  100  comprises a cone brake located in the vicinity of the outer end of the input shaft  44 . A cone brake is preferred because it is simple in design, non-intrusive to the gearbox  40 , and easily incorporated into an existing gearbox design. It also does not expose the internal components of the gearbox  40  to contamination by dust or water. 
     Referring to  FIGS. 2–5 , the cone brake  100  includes a cup  102 , a cone  104 , and return spring  106  that biases the cone  104  to an engaged position. The cup  102  surrounds the input shaft  44  and is press-fit or otherwise mounted in a counterbore  107  in the outer gearbox cover  64 . The cone  104  is keyed to or otherwise mounted on the outer end of the input shaft  44 . It preferably includes a powdered metal cone. The currently preferred metal is FN-0208-105HC, which has an apparent hardness of RC31. The outer periphery of the cone  104  has a cylindrical outer axially surface portion  108  and an axially inner frusto-conical portion  110  that is tapered inwardly from an outer end thereof to an inner end thereof. The surface of the tapered inner frusto-conical portion  10  is configured to rub against a correspondingly tapered surface  112  on the inner periphery of the brake cup   102  to apply the brake  100 . The return spring  106  is configured to bias the shaft  44  and, hence, the cone  104  toward the engaged position of  FIG. 4 , thereby assuring automatic brake engagement upon the cessation of torque transfer to the input shaft  44 . A variety of springs could be used and act on a variety of different components so as to directly or indirectly perform the desired biasing effect. In the illustrated embodiment, the spring  106  comprises a spiral wave spring that acts on the bearing  68 . It is located in a chamber formed outward of the bearing  68  so as to rest against a radial step  114  of the outer cover  64  at its outer end and against a spacer  116  at its inner end. Finally, a seal  118  is disposed outwardly of the spring chamber for sealing the interior of the housing  42  surface from the environment while still permitting the shaft  44  to move axially relative to the housing  42 . 
     4. Operation of Trowel 
     During normal operation of the trowel  10 , torque is transferred to the gearbox input shaft  44  from the engine&#39;s output shaft, the clutch, and the drive train. The worm  80  then transfers torque to the worm gear  82  which, in turn, drives the output shaft  26  to rotate counterclockwise, thereby driving the rotor  12  to rotate. The reaction forces imposed on the teeth of the worm  80  by the teeth of the worm gear  82  drives the input shaft  44  and bearing  68  to the position illustrated in  FIG. 3  to drive the cone  104  away from the cup  102 , thereby releasing the brake  100  and permitting unobstructed rotation of the input shaft  44  relative to the gearbox housing  42 . At some time, cessation of torque delivery to the input shaft occurs, either because the operator shuts down the engine or throttles it back to release the clutch or, in the case of more sophisticated systems, because a sensor  such as an accelerometer or a gyroscope detects actual or imminent loss of operator control and generates a signal to disable the drive system. The reaction forces driving the shaft  44  axially to the left in the drawings therefore are removed, permitting the shaft  44  to move to the right or to the brake engaged position of  FIG. 4 , thereby inhibiting rotation of the cage  14  and handle  20 . The cone  104  meshes with the cup  102  to engage the brake  100  as a result of this movement. This movement is driven at least in part by the return spring  106 . However, and as a significant aspect of the invention, it may also be driven in part by backdrive force imposed when the rotating output shaft  26  applies torque to the worm  80  through the worm gear  82 . The magnitude of this backdrive force is dependent upon the backdrive torque. Therefore, the magnitude of braking forces generated by the brake  100  are dependent on the magnitude of the backdrive torque, hence resulting in more effective braking when it is most critical. 
     Both brake engagement and release occur fully automatically. As a result, operator input to both actions is completely unnecessary. The operator therefore is free to perform the more desired and less fatiguing steering and control functions. 
     Many changes and modifications could be made to the invention without departing from the spirit thereof. For instance, while the brake component of the drive train is preferably located within the gearbox  40 , that need not be the case. Moreover, if the brake is incorporated into the gearbox  40 , it could be used with drives other than worm drives, so long as the drive has a torque transfer system having a component that experiences thrust upon the transfer or drive torque thereto. Virtually any drive component having angled teeth, i.e., ones that extend at an angle that is offset from the radial, could suffice.  
     Still other changes that could be made to the invention without departing from the spirit thereof will become apparent from the appended claims.