Abstract:
A hydraulic control circuit comprises a power unit, a central processing unit, at least one feedback sensor, a valve manifold, and two or more hydraulic lifting cylinders interconnected with miscellaneous hydraulic hoses and electrical wiring. Basic lifting is regulated by a flow divider unit configured to distribute a flow of pressurized hydraulic fluid pumped from a fluid reservoir through the valve manifold to each of the lifting cylinders during a lifting operation. To compensate for any imbalance between the lifting cylinders, the central processing unit monitors the movement of the lifting cylinders, and is configured to divert, through a three-way valve in the valve manifold, an additional portion of the pressurized fluid flow to a lagging lifting cylinder. During decent operations, the central processing unit extracts an additional portion of the fluid return flow through the three-way valve from a lagging lift cylinder, such that at all times during either lifting or decent operations, each lifting cylinder and a supported runway are disposed in a substantially parallel configuration.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     None. 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not Applicable. 
     BACKGROUND OF THE INVENTION 
     The present invention relates generally to a control for a hydraulic fluid distribution system utilized in an automotive vehicle lift rack having a pair of supporting runways each of which are elevated by at least one associated fluid actuated ram supplied from a common fluid reservoir, and more specifically, to a hydraulic fluid control circuit capable of compensating for uneven hydraulic fluid flow rates to and from the fluid actuated rams to maintain the runways in a level configuration under offset loading conditions at all times. 
     Traditionally, automotive repair shops and garages employ post-style hydraulic lifts having large hydraulic rams located below the floor of the garage to lift a pair of runways upon which a vehicle undergoing service is parked. These systems require excavations below the floor of the repair shop for the installation of the hydraulic ram or post, as well as for a hydraulic fluid reservoir and the associated plumbing. Due to increased regulations by the U.S. Environmental Protection Agency relating to the storage of potentially toxic fluids such as hydraulic fluid below ground, the trend in repair shops has been to utilize ground-level lift systems which do not require any below ground excavation or fluid storage. 
     Ground level lift systems, such as shown in U.S. Pat. No. 5,199,686 for “Non-Continuous Base Ground Level Automotive Lift System” and U.S. Pat. No. 5,096,159 for “Automotive Lift System”, both to Fletcher are examples of parallelogram-style ground level automotive lifts. A comparable design is seen in U.S. Pat. No. 5,102,898 for “Control System for Vehicle Lift Racks” to Tsymberov. In the &#39;686 and &#39;898 systems, the pair of runways upon which the automotive vehicle undergoing service is parked are supported in an elevated position by separate hydraulic fluid rams or lifting elements. These rams are pressurized from a common hydraulic fluid circuit connected to an above-ground fluid reservoir. Several factors must be taken into consideration when designing and utilizing parallelogram lifts such as these. For example, there is a critical need to maintain each of the lift runways in a substantially parallel configuration at all times, despite the occurrence of uneven or offset loading conditions, as well as the need to maintain substantially the same fluid flow to each of the supporting hydraulic fluid rams during the raising or lowering of the lift runways. 
     As is shown in the &#39;898 Tsymberov patent an even fluid distribution between the two or more hydraulic fluid rams can be achieved to some degree through the simple use of a flow divider/combiner valve, however, this is generally an inaccurate method of ensuring an even fluid distribution during offset loading conditions. The &#39;686 Fletcher patent discloses the use of a complex arrangement of hydraulic control circuits and flow dividers utilized to coordinate the raising and lowering of the adjacent runways of a parallelogram lift, and to compensate for uneven flow rates of hydraulic fluid to each of the hydraulic rams. Specifically, the &#39;686 patent employs a system control valve and a proportioning valve to control hydraulic fluid flow into and out of each supporting hydraulic fluid ram through both upper and lower ports. These circuits in the &#39;686 patent utilize the arrangement of flow dividers and combination valves to either withdraw hydraulic fluid from a hydraulic cylinder which is elevating faster than another, or to withdraw additional hydraulic fluid from a hydraulic cylinder which is descending slower than another. In addition to incorporating a number of expensive components, these hydraulic fluid circuits often require lengthy calibration procedures to ensure that they are capable of maintaining a pair of runways in a substantially parallel configuration throughout the vertical operational range of the lift, even when loaded with an offset weight distribution. 
     Accordingly, there is a need to improve the design of the hydraulic fluid control circuits associated with these increasingly popular ground level lift systems and other lift systems having two or more independent lifting elements such that the circuits are inexpensive to manufacture, do not require extensive calibration and testing prior to operational installation, and are capable of maintaining the runways or lifting elements of the lift in a substantially parallel configuration throughout a vertical lift range despite severe offset loading conditions during the raising and lowering cycles. 
     BRIEF SUMMARY OF THE INVENTION 
     Among the several objects and advantages of the present invention are: 
     The provision of a hydraulic control circuit configured to regulate hydraulic fluid flow from a fluid reservoir to two or more hydraulic rams or cylinders configured to support the adjacent and independent runways of an automotive lift in a substantially parallel configuration during vertical elevation and lowering cycles; 
     The provision of the aforementioned hydraulic control circuit which is configured to regulate hydraulic fluid flow to and from the hydraulic rams or cylinders to maintain the lift runways in substantially parallel positions at all times during uneven or offset loading conditions; 
     The provision of the aforementioned hydraulic control circuit which is configured to regulate hydraulic fluid flow to and from the hydraulic rams to maintain the lift runways to within a predetermined and adjustable range of vertical variation from parallel at all times under an extreme offset loading condition; 
     The provision of the aforementioned hydraulic control circuit wherein the hydraulic fluid flow operation is controlled by a central processing unit; 
     The provision of the aforementioned hydraulic control circuit wherein the central processing unit is responsive to the vertical position of the pair of adjacent and independent lift elements to adjust hydraulic fluid flow to associated hydraulic rams; 
     The provision of the aforementioned hydraulic control circuit wherein the central processing unit is responsive to the angular positioning of a pair of laterally adjacent and independent lift element support legs to adjust hydraulic fluid flow to associated hydraulic rams; 
     The provision of the aforementioned hydraulic control circuit wherein the central processing unit is responsive to signals received from sensors secured to pivot points on laterally adjacent and independent lift element support legs to adjust hydraulic fluid flow to associated hydraulic rams; 
     The provision of the aforementioned hydraulic control circuit wherein the central processing unit operates a three-way, two-position valve to independently alter hydraulic fluid flow to the individual hydraulic rams; 
     The provision of the aforementioned hydraulic control circuit wherein an alternative embodiment the central processing unit operates a three-way, three-position valve to independently alter hydraulic fluid flow to the individual hydraulic rams; 
     The provision of the aforementioned hydraulic control circuit wherein fluid flow to a lagging hydraulic ram is increased during elevation of the automotive lift runways to maintain the pair of runways in a substantially parallel configuration during vertical movement; 
     The provision of the aforementioned hydraulic control circuit wherein fluid flow from a lagging hydraulic ram is increased during lowering of the automotive lift to maintain the pair of runways in a substantially parallel configuration during vertical movement; 
     The provision of the aforementioned hydraulic control circuit wherein the central processing unit is configured to include automatic operation of safety features; 
     The provision of the aforementioned hydraulic control circuit wherein the central processing unit is configured to cease movement of the automotive lift if the lift runways are detected to vary by more than a predetermined amount from a parallel configuration; 
     The provision of the aforementioned hydraulic control circuit wherein the central processing unit is configured to maintain the runway surfaces parallel to within a predetermined vertical variation under an offset loading condition; 
     The provision of the aforementioned hydraulic control circuit wherein it may be readily adapted for use in controlling the operation of any two or more independent hydraulically actuated components utilizing a common fluid source wherein there is a need to maintain a relative displacement between the actuated components; and 
     The provision of the aforementioned hydraulic control circuit wherein a low cost, commonly available fluid flow divider component is utilized to provide a low cost, high accuracy fluid control circuit. 
     Briefly stated, the hydraulic control circuit of the present invention for use with a hydraulic lift system having two or more independent lifting elements. More specifically, a ground-level automotive lift system having two independent vehicle runways. The control circuit comprises a central processing unit, a power unit manifold, an auxiliary valve manifold, two or more hydraulic lifting cylinders, and at least one feedback sensor associated with the automotive lift system, all of which are interconnected with miscellaneous hydraulic hoses and electrical wiring to regulate the elevation or lowering of the automotive lift. Basic lifting or elevation is regulated by a flow divider/combiner valve unit configured to distribute a flow of pressurized hydraulic fluid pumped from a fluid reservoir through the auxiliary valve manifold to each of the hydraulic lifting cylinders or rams during a lifting operation. To compensate for any imbalance between the rate of extension of the lifting cylinders, the central processing unit monitors the movement of each of the lifting cylinders, and is configured to divert, through a valve in the auxiliary valve manifold, an additional portion of the pressurized hydraulic fluid flow to a lagging lifting cylinder to increase the rate of ascent. During decent or lowering operations, the central processing unit again compensates for any imbalances in the rate of retraction for each hydraulic cylinder by extracting an additional portion of the hydraulic fluid return flow from a lagging lift cylinder through the three-way valve, such that at all times during either lifting or decent operations, each hydraulic ram or cylinder and a supported runway lift are disposed in a substantially horizontal planar configuration. If the central processing unit detects a vertical displacement variation greater than a predetermined setting between the supported runways or lifting elements of the automotive life, the operations are halted until the discrepancy can be corrected. 
     The foregoing and other objects, features, and advantages of the invention as well as presently preferred embodiments thereof will become more apparent from the reading of the following description in connection with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     In the accompanying drawings which form part of the specification: 
     FIG. 1 is a schematic pressure fluid diagram showing the organization of the components in the hydraulic control system. 
    
    
     Corresponding reference numerals indicate corresponding parts throughout the several figures of the drawings. 
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The following detailed description illustrates the invention by way of example and not by way of limitation. The description clearly enables one skilled in the art to make and use the invention, describes several embodiments, adaptations, variations, alternatives, and uses of the invention, including what is presently believe to be the best mode of carrying out the invention. 
     Turning to FIG. 1, a schematic pressure fluid diagram showing the organization of the components in the fluid control system of the present invention is illustrated. The fluid control system shown is the preferred embodiment, and comprises four main components, a power unit manifold  10 , an bi-directional fluid flow divider/combiner circuit or auxiliary valve manifold  12 , a left fluid ram or lifting cylinder  14 , and a right fluid ram or lifting cylinder  16 . The lifting cylinders  14 ,  16  are preferably secured to an automotive vehicle lift system (not shown) so as to vertically elevate two or more vehicle support runways or support members (not shown) to provide access to the underside of an automotive vehicle for service thereof. These components are interconnected with miscellaneous pipes and hoses in fluid communication to form a fluid circuit. In the preferred embodiment the system is utilized with a hydraulic fluid, although alternative fluids having the necessary compression and flow characteristics may be employed. 
     It will be readily recognized that additional pairs of hydraulic rams may be added to the control system of the present invention as required for the particular lifting application to which the system will be applied without deviating from the scope and concept of the invention. The power unit manifold  10  is located upstream from a hydraulic fluid reservoir  18 . During operation, hydraulic fluid is drawn through an intake line  19  from the reservoir, through a filter  20 , and pumped into the power unit manifold  10  along main line  21  by means of a pump  22  driven by an electric motor  24 . The power unit manifold includes on the main line  21  a reverse flow check valve  26 , a pressure relief valve  28  interconnected to the main line  21  downstream of the reverse flow check valve  26 , and a two-way flow return valve  30  located upstream of the reverse flow check valve  26 . 
     In a lifting cycle or pressurized operation of the control system, hydraulic fluid withdrawn from the reservoir  18  by suction generated at pump  22  passes through the reverse flow check valve  26  and into the auxiliary manifold  12 . The reverse flow check valve  26  prevents the hydraulic fluid from returning to the pump  22  in the reverse direction. In the event pressure is detected in the hydraulic control circuit which exceeds a preset pressure relief setting, the pressure relief valve  28 , located between the reverse flow check valve  26  and the motorized pump  22  will open, diverting a portion of the hydraulic fluid flow from the main line  21  back to the reservoir  18  along a return line  29 , rather than allowing it to continue through the rest of the system. 
     During a lowering cycle of the control system, hydraulic fluid withdrawn from the left hydraulic lifting cylinder  14  and the right hydraulic lifting cylinder  16  returns to the fluid reservoir  18  after passing through the auxiliary manifold  12 , through the two-way flow return valve  30  and return line  31 . The returning fluid is prevented from returning to the reservoir  18  through the pump  22  by the reverse flow check valve  26  on the main line  21 , and hence diverted to the two-way flow return valve  30 . Normally in a reverse flow restricted position  30 A, as shown in FIG. 1, the two-way flow return valve  30  is opened to a second position  30 B by actuation of a solenoid  32  during a lowering cycle to permit the returning hydraulic fluid on return line  31  to flow unrestricted into the reservoir  18 . 
     When the control system is actuated to provide lift to the left and right hydraulic lifting cylinders  14 ,  16  pressurized hydraulic fluid exiting from the power unit manifold  10  travels through a connecting hose  34  to the auxiliary manifold assembly  12 . Upon entering the auxiliary manifold assembly  12 , the fluid passes through a flow control valve  36  in the unmetered direction. The fluid exits the flow control valve  36  through line  37 , and enters a fluid proportioning valve or flow divider/combiner valve  38  through port  38 A where the fluid flow is split approximately equally to each port  38 B and  38 C. Fluid exiting the flow divider/combiner valve  38  through the port  38 B enters branch line  39 , and passes through a first two-way valve  40  in an unchecked direction, exiting the auxiliary manifold  12  and passing through a connecting hose  41  to a velocity fuse  42 . The hydraulic fluid passes through the velocity fuse  42  in an unmetered direction to enter the left hydraulic lifting cylinder  14 , exerting an expansion force thereon. Similarly, fluid exiting the flow divider/combiner valve  38  through the port  38 C enters branch line  43  and passes through a second two-way valve  44  in an unchecked direction, exiting the auxiliary manifold  12  and passing through a connecting hose  45  to a velocity fuse  46 . The hydraulic fluid passes through the velocity fuse  46  in an unmetered direction to enter the right hydraulic lifting cylinder  16 , exerting an expansion force thereon. 
     Due to proportioning inaccuracy of the flow divider/combiner valve  38 , the flow of hydraulic fluid under pressure through the valve  38  may not be split exactly in equal ratios to the ports  38 B and  38 C, thereby causing an unequal amount of hydraulic fluid to be diverted to either the left or right hydraulic lifting cylinder  14 ,  16 . Such an uneven flow of hydraulic fluid causes one hydraulic lifting cylinder to expand at a rate different from the other, resulting in an uneven ascension of the automotive lift runways supported thereon. This condition may be further exaggerated if the automotive lift runways are not carrying an equal load. 
     To compensate for unequal flow distribution of hydraulic fluid during a lifting cycle, a small amount of hydraulic fluid is extracted from fluid line  37  in the auxiliary valve manifold  12  between the flow control valve  36  and the flow divider/combiner valve  38 . The extracted hydraulic fluid is routed through a controlling orifice  48  and directed by a bi-directional fluid flow diverting valve, or three-way, two-position valve  50  to the branch line  39 ,  43  on the output side of the flow divider valve  38 . Alternatively, valve  50  may be replaced with a three-way, three-position valve having a blocked flow position. The amount of fluid bypassing the flow divider valve  38  is controlled by the size of the opening in the control orifice  48 , which may be altered to provide a desired fluid flow. The branch line  39 ,  43  to which the fluid is routed is selected for the hydraulic lifting cylinder  14 ,  16  which is observed to be lagging though feedback sensors  52  and  54 . The feedback sensors  52 ,  54  are located on the lift structure (not shown) to which the hydraulic lifting cylinders  14 ,  16  are connected. 
     In the preferred embodiment, the feedback sensors  52  and  54  are Hall effects sensors which translate angular displacement from a rest position into a proportional voltage signal. Placing the sensors at pivot points in the lift structure (not shown) permits the sensors to observe the change in height of the lift structure by sensing the altered geometric relationships between elements of the lift structure. Those skilled in the art will readily recognize that a variety of sensors having sufficient sensitivity may be employed to observe variations in the geometry of the lift structure. For example, linear displacement sensors could be employed to directly measure the extension and retraction of the hydraulic lifting cylinders  14 ,  16 . Signals from the feedback sensors  52 ,  54  are routed to an electronic control unit or central processing unit  56  which is configured, in the preferred embodiment, to digitally convert the voltage signals representing angular displacements at the sensors into changes in elevation of the runway lift structures as small as 0.125 inches. Alternative sensors with appropriate sensitivity may be utilized to detect a different amount of elevation variation, depending upon the particular application of the lift structure. 
     The central processing unit is further configured to detect whenever a vertical height variation of at least 0.25 inches in the preferred embodiment occurs between the vertical positions of the runway lift structures to which the hydraulic lifting cylinders  14 ,  16  are connected. The degree of detected variation may be adjusted at the central processing unit to allow for either coarser or finer adjustments to be made. Upon detecting a selected variation condition, the central processing unit actuates a solenoid  58  to divert a controlled portion of the fluid flow through the three-way valve  50  to the branch line  39 ,  43  connected to the lagging hydraulic lifting cylinder. Once the lagging hydraulic lifting cylinder  14 ,  16  receiving the diverted fluid flow extends sufficiently far to become the leading ram as detected by the feedback sensors  52 ,  54 , the three-way valve  50  is switched by the central processing unit  56  to redirect the flow of extracted hydraulic fluid to the second hydraulic lifting cylinder  14 ,  16  which is now the lagging lifting cylinder. This process continues throughout the entire lifting cycle of the hydraulic control circuit. 
     In the unlikely event the central processing unit  56  detects a condition wherein the feedback sensors  52 ,  54  register a vertical position variance between the left and right hydraulic lifting cylinders  14 ,  16  exceeding a predetermined setting, a safety protocol will shut down operation of the hydraulic circuit until the condition is corrected. Such conditions could be caused by a ruptured hydraulic line or a blockage in the fluid circuit, with continued operations leading to a general failure of the system. 
     When the control system is actuated to lower the left and right hydraulic lifting cylinders  14 ,  16 , the flow of hydraulic fluid through the system is substantially reversed. To permit hydraulic fluid to exit the hydraulic lifting cylinders  14 ,  16 , the central processing unit  56  actuates solenoids  32 ,  60 , and  62  simultaneously to shift each of the two-way valves  30 ,  40 , and  44  from the checked flow positions to the free flow return positions. The force of gravity acting on the mass of the runway lift structures (not shown) supported by the hydraulic lifting cylinders  14 ,  16  will cause hydraulic fluid to exit the lifting cylinders  14 ,  16  through velocity fuses  42 ,  46 . The velocity fuses  42 ,  46  meter the rate of fluid flow exiting the hydraulic lifting cylinders  14 ,  16 . If the flow rate exceeds a predetermined amount, due to a ruptured hose for example, the velocity fuses  42 ,  46  will completely shut off all fluid exiting the hydraulic lifting cylinders  14 ,  16 , locking the runway lift structure (not shown) in a safe condition. 
     Once the return flow of hydraulic fluid passes through the velocity fuses  42 ,  46 , and the two-way valves  40 ,  44  in their free flow positions, it re-enter the flow divider/combiner valve  38  through ports  38 B and  38 C. Inside the flow divider/combiner valve  38 , the two hydraulic fluid flows from the left and right hydraulic lifting cylinders  14 ,  16  are recombined into a single fluid flow in approximately equal ratios. The combined hydraulic fluid flow then exits the flow divider valve  38  through port  38 A into line  37 , and passes through the flow control valve  36  in the metered direction towards the power unit manifold  10 . The flow control valve  36  is pressure compensated to regulate the speed at which the hydraulic fluid flows, thereby limiting the rate of descent for the hydraulic lifting cylinders  14 ,  16  regardless of the load carried thereby. Once through the flow control valve  36 , the hydraulic fluid enters the power unit manifold  10 , and is diverted by the reverse flow check valve  26  along return line  31  to the two-way valve  30 , now in the free flow position, returning to the fluid reservoir  18 . 
     As with expansion of the hydraulic lifting cylinders  14 ,  16 , the inaccurate nature of the flow divider/combiner valve  38  prevents the two separate hydraulic fluids streams exiting from each of the hydraulic lifting cylinders  14 ,  16  from combining in exactly equal proportions as the fluid returns to the fluid reservoir  18 . This unequal combination of the hydraulic fluid streams in the flow divider/combiner valve  38  causes one of the hydraulic lifting cylinders  14 ,  16  to lag behind the other during the descent cycle, exhibiting a vertical variance between the supported runway lift structures (not shown). This variance is detected at the central processing unit  56  from signals received through the feedback sensors  52 ,  54 . 
     To compensate for the unequal combination of the hydraulic fluid streams at the flow divider/combiner valve  38 , resulting in uneven descent rates for the automotive lift members, the central processing unit switches the three-way valve  50  to allow a portion of fluid from the lagging hydraulic lifting cylinder  14 ,  16  to bypass the flow divider valve  38  and return to the fluid reservoir  18  through the control orifice  48 . The amount of fluid bypassing the flow divider valve  38  is controlled by the size of the opening in the control orifice  48 , which may be altered to provide a desired fluid flow. Once sufficient hydraulic fluid has been withdrawn from the lagging hydraulic lifting cylinder  14 ,  16  such that it is now in a leading position, the central processing unit signals the solenoid  58  to switch the three-way valve to the second position, draining fluid from the second, now lagging, hydraulic lifting cylinder  14 ,  16 . This process repeats until the descent cycle is completed. As in the ascent cycle, the central processing unit  56  is preferably configured to actuate the solenoid  58  upon detecting a vertical variance of only 0.25 inches between the runway lift surfaces secured to the hydraulic lifting cylinders  14 ,  16 , however, the amount of vertical variance allowed may be adjusted to suit the application. Additionally, should the central processing unit  56  detect a predetermined vertical variance between the lifting elements any time during a descent cycle, the process will be halted as a safety measure. 
     In addition to running the pump  22  and regulating the bypass hydraulic fluid flows in response to readings obtained from the feedback sensors  52 ,  54 , the central processing unit  56  is configured to perform a variety of functions, including calibration of the feedback sensors  52 ,  54  upon start-up, and regulation of the runway lift structure (not shown) minimum and maximum positions. For example, if the lift structure (not shown) were to comprise a pair of runway ramps for use in servicing an automotive vehicle, a maximum lift height for the lift structure could be set in the central processing unit  56  such that a vehicle placed on the runway ramps would not contact the ceiling or other overhead structures when elevated for servicing. Once the central processing unit detects that the hydraulic lifting cylinders  14 ,  16  have extended such that the lift structure (not shown) has reached the predetermined maximum lift height, the central processing unit  56  signals the motor  24  and solenoid  58  to stop operation. Additionally, upon detecting a certain minimum elevation, the central processing unit  56  could activate a number of auxiliary lights (not shown) secured to the lift structure (not shown). 
     The present invention additionally provides a method for regulating the ascent and descent of an above ground automotive vehicle lift system, particularly of the type having two independently articulated vehicle support members or runways actuated by a fluid pressure system. To raise the vehicle support runways, a fluid under pressure is supplied from a common fluid source to the fluid driven lifting components through a fluid circuit. Within the fluid circuit, the fluid flow is divided into substantially equal portions between each of the fluid driven lifting components. Due to the inaccurate nature of fluid proportioning circuits, and any offset loading between the vehicle support runways, one of the lifting components is likely to elevate an associated vehicle support runway at a rate greater than the other, resulting in a variation in vertical displacement. By observing any variations in vertical displacement exceeding a predetermined amount through sensors, additional fluid may be directed from the common fluid source to the lifting components which are observed to be lagging during the ascending motion. 
     During descending motion, or the lowering of the vehicle support runways, variations in the vertical displacement between the vehicle support runways is again observed through the sensors. By sensing any variations in vertical displacement exceeding a predetermined amount, additional fluid may be routed to the common fluid source from the lifting components which are observed to be lagging during the descending motion. 
     In view of the above, it will be seen that the several objects of the invention are achieved and other advantageous results are obtained. Those skilled in the art of hydraulic circuit design will readily recognize that a variety of valves other than those described in the preferred embodiment may be employed without changing the scope of the invention. For example, valves may utilize one solenoid and a compression spring to provide actuating movement, or may utilize two solenoids in a push-pull configuration. Similarly, it will be readily recognized that common substituting components are available which will function equally well. For example, the control orifice  48  may be replaced by an adjustable needle valve or a flow limiter without changing the scope of the invention. As various changes could be made in the above constructions without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.