Method and power transmission system for operating a road planar machine

The method and power transmission system of this invention for driving a road planar machine has the cutter unit in a positive mechanical drive with the drive shaft of a machine power unit through a hydraulically actuated clutch mechanism. The wheel traction units for the machine are continuously driven from the power unit by hydraulic motors operated by variable volume displacement pumps. A control system for retaining the power unit at a set rotational speed required for a predetermined cutting or planing load is responsive to a drop in the set rotational speed, resulting from the cutting unit encountering an overload, to actuate the variable displacement pumps to decrease the forward speed of the machine without appreciably reducing the rotational inertia force of the cutting unit. If the cutting unit overload is of a short duration or momentary the set rotational speed of the power unit is resumed. If the overload, as observed by the machine operator after the occurrence of the drop in the set rotational speed of the power unit, appears to be severe or prolonged, the control system may be manually discontinued and the machine manually controlled until the cutting unit overload has been removed at which time the control system may again be placed into operation.

BACKGROUND OF THE INVENTION 
Planar machines and saw cutting machines for road maintenance work 
generally drive the cutter heads and saws in a rather conventional manner 
directly from a power unit or through a hydraulic pump and motor. In U.S. 
Pat. No. 2,311,891, a road groove cutting tool is directly driven from a 
separate engine provided for such purpose, through a transmission 
mechanism and the traction units are driven through a slip-clutch from 
another engine. Thus, if the cutting tool is held back by road conditions, 
the clutch will slip to arrest the advance travel of the machine so as to 
reduce the load on the cutting tool engine. U.S. Pat. No. 2,817,275 
discloses a planar machine in which the cutter head is chain and sprocket 
driven from an engine through a clutch mechanism. The machine is manually 
propelled so that advancement of the cutter is readily controlled relative 
to cutting conditions. In the roadway slotting machine of U.S. Pat. No. 
3,321,250, the cutter is belt driven from an engine which is also used to 
drive traction wheels through means including a hydraulic pump to motor to 
differential arrangement. The road groove cutter in U.S. Pat. No. 
3,333,897 is in a direct engine drive connection while in U.S. Pat. No. 
4,139,318, the cutter head and traction units are driven in a conventional 
manner. 
In the prior art devices having a belt driven cutter or a cutter having a 
slip-clutch connection with the power unit, the cutter rotational speed 
and inertia force are appreciably decreased or stopped when an obstruction 
is encountered. The cutting operation is thus either stalled or takes 
place at a slow speed with resultant loss of time and increased expense. 
Where the advance travel of the machine is continued concurrently with a 
positive drive of the cutter head as in U.S. Pat. No. 4,139,318, for 
example, the cutter head is susceptible not only to heavy tooth damage, 
but also damage to the power unit and/or its drive system. 
SUMMARY OF THE INVENTION 
The invention provides an improved method and system for driving a road 
planar machine and a cutter head or unit therefor from a single power unit 
to remove a top layer of a roadway efficiently and with a minimum of lost 
time. The rotational speed of the engine is monitored through a control 
system which, in response to a drop in a set engine rotational speed 
caused by an overload condition encountered by the cutter head, 
concurrently slows the advance of the machine, while maintaining a 
substantially constant inertia force for the cutter head. The cutter head 
thus continues to operate with full inertia force against the obstructive 
condition. If the overload condition is removed, the set engine speed is 
resumed. The planing operation is thus substantially continuous so as to 
reduce shutdown time and machine damage and maintenance to a minimum 
without requiring constant observation to road conditions by the machine 
operator.

DESCRIPTION OF THE INVENTION 
Referring to FIG. 1 of the drawings, there is shown a planar type machine 
15 for removing a predetermined top layer of a paved road surface, that 
embodies the power transmission system of this invention. The machine 15 
has a main frame or chassis 16 for a front end cab section 17, an 
intermediate power section 18 and a rear fuel and accessory section 19. A 
rotary planar or cutting unit 21 is located below the power section 18 and 
between a pair of front steerable traction wheels 22 and rear traction 
wheels 23. The various manually operated and controlled actuating elements 
for operating the machine 15 are conveniently located within the cab 
section 17. 
In machines of this general type, the rotary cutting unit may be driven 
from a power unit by means including a slip clutch, or by a hydraulic 
system which includes a bypass or relief valve to prevent damage against 
any overload imposed on the cutting unit. In either instance, when the 
cutting unit engages an obstruction, so as to set up an overload condition 
that exceeds the adjusted setting of the slip clutch or relief valve, the 
centrifugal inertia force of the rotary cutting unit immediately falls off 
or is reduced so as to aggravate the imposed load on the cutting unit by 
effecting a slow down in or a stopping of the road planing operation. This 
is especially inconvenient and time consuming when the cutting unit is 
repeatedly forced against the obstruction at variable speeds caused by the 
erratic or vacillating power delivered to the cutting unit. Additionally, 
where the cutting unit is hydraulically operated, the repeated operation 
of a relief valve results in the heating of the oil in the hydraulic 
system so as to further impede a resumption of normal machine operation. 
Where the power unit is in a direct mechanical drive connection with the 
cutting unit, appreciable damage to the cutting unit or machine may occur 
when an obstruction is encountered due to the continued advance of the 
machine into the obstruction with full power applied on the cutting unit. 
Extreme care and attention to the road surface is therefore required by 
the machine operator to prevent machine damage and possible personal 
injury. 
To overcome these objections, the power section 18 includes a power unit 24 
of diesel engine type which is operatively connected through a flexible 
coupling 26 (FIG. 4) to a transmission mechanism 27 that has a power take 
off shaft 28 gear connected in a direct driving relation with the cutting 
unit 21 through a hydraulically actuated clutch mechanism 31. When the 
clutch 31 is engaged, the cutting unit 21 is driven directly from the 
transmission mechanism 27, and in turn from the engine 24, at a reduced 
rotational speed relative to the rotational speed of the take off shaft 28 
through a gear reduction unit 32. The power take off shaft of the gear 
reduction unit 32 is connected in a one-to-one chain and sprocket drive 
connection 33 with the cutting unit 21. 
The power shaft 28 also directly drives through a gear system 29 a pair of 
variable volume displacement pumps 34 and 36 of like construction, but 
reversely rotated in operation, having associated hydraulic motors 37 and 
38, respectively, for driving the front and rear traction wheels 22 and 
23, also respectively (FIGS. 2, 3 and 4). In one embodiment of the 
invention, the pumps 34 and 36 are heavy duty Sundstrand pumps (Models 
20-2065 and 20-2074), available from Sundstrand Hydro-Transmission, Ames, 
Iowa, a division of Sundstrand Corporation. It is seen, therefore, that 
when the clutch unit 31 is engaged, the rotational speed of the cutting 
unit and linear advance of the machine are directly responsive to the 
rotational speed of the engine power unit 24. 
The gear system 29 includes a jack shaft 30 to drive a pump 41 for 
supplying oil under pressure to the hydraulically actuated components of 
the machine such as the previously referred to clutch mechanism 31 and, as 
will later appear, hydro-transmission valves in the variable volume 
displacement pumps 34 and 36. To automatically retard the advance of the 
machine 15 concurrently with maintaining the centrifugal inertia force on 
the cutting unit 21 when there is a drop in the normal operating 
rotational speed of the engine 24, resulting from an overload condition 
imposed on the cutting unit, there is provided a constant engine speed 
control system. 
The constant engine speed control system (FIG. 5) includes a twelve volt 
battery 42 as the electrical power source. A Honeywell W883 speed control 
amplifier 43, available from Honeywell, Inc. of Minneapolis, Minnesota, 
compares the engine speed indicated by a Honeywell SB100A magnetic pulse 
pickup 44 with a speed set point potentiometer 46 which is set at the 
normal operating speed of the engine for a normal cutting load on the 
cutting unit 21. So long as the engine speed is equal to or greater than 
the speed set point, the output signal from the speed control amplifier 43 
is approximately ten volts. This output voltage is used to power a 
normally manually operated control handle 47 which forms part of the 
variable volume displacement pumps 34 and 36. This handle 47 operates a 
hydro-transmission valve 48 on each pump 34 and 36 for actuating a swash 
plate 49 operatively associated with pistons 51, all of which form part of 
the commercially available pumps 34 and 36. A position of the control 
handle 47 generates a variable current signal which drives the 
hydro-transmission valves 48 to vary the operating angle of the swash 
plate 49 relative to its axis of rotation, and in turn the volume 
displacement of the pumps 34 and 36. It is seen, therefore, that with a 
fixed voltage to power the control handle 47, each handle position defines 
a specific operating angle of the swash plates 49 and a definite volume 
displacement by the pumps 34 and 36. 
When the engine speed falls below the desired set point speed, due to an 
overload on the cutting unit 21, the ten volt output signal from the speed 
control amplifier 43 starts to reduce or drop. This drop in the voltage 
supply to the control handle 47 results in a corresponding drop in the 
current to the hydro-transmission valves 48 and a destroking of the swash 
plates 49, i.e., a reduced stroke of the pistons 51 associated therewith 
in the variable volume displacement pumps 34 and 36 to reduce the volume 
of oil supplied to the traction motors 37 and 38, respectively. This 
operation of the control system acts to concurrently reduce the load on 
the engine while maintaining the rotational inertia force of the cutting 
unit 21 to overcome the encountered overload. When the imposed load on the 
cutting unit has been eliminated, the control system automatically takes 
over for return of the engine to the set point speed. 
Should there occur a failure of the speed control amplifier 43 or the 
encountering of a cutting unit obstruction greater than can be eliminated 
by the reduced machine speed, the control system does not provide for a 
shut down of the machine. In this event, an auto-manual switch 52 is used 
to provide for a manual control of the machine from the constant twelve 
volt battery source. The machine operator merely manipulates the switch 52 
from `automatic` to `manual` and continues the operation of the machine 
independently of the speed control system. When normal operation of the 
machine is to be resumed, the control system is reinstated by returning 
the switch 52 to its `automatic` position. 
The constant engine speed control system thus functions primarily as a load 
limiting system which prevents excessive loads being placed on the engine 
24. Thus, when the engine speed starts to drop due to excessive loading, 
the system automatically cuts the load by reducing the advance of the 
machine and maintaining the rotational inertia of the cutting unit. 
As shown in FIG. 6, the machine 15 is being operated with the cutting unit 
21 removing a layer 53 of the roadway and approaching an obstruction 54. 
During this time the control system is operating with the ten volt output 
signal from the amplifier 43. On encountering the obstruction 54 (FIG. 7) 
the engine speed is reduced with a concurrent drop in the voltage to the 
hydro-transmission valves 48 and reduction in the volume of oil supplied 
to the traction motors 37 and 38. This slow down operation of the machine 
continues until the obstruction 54 has been removed, as shown in FIG. 8, 
at which time normal operation of the machine is resumed. 
Although the invention has been described with respect to a preferred 
embodiment thereof, it is to be understood that it is not to be so limited 
since changes and modifications can be made therein which are within the 
full intended scope of this invention as defined by the appended claims.