Brake control system with metering pump relief

This invention relates to a brake control system (10) for controlling engagement and disengagement of a spring applied pressure released brake (11) of a winch and includes an orifice (39) and a relief valve (37) connected in series to the outlet port of a metering pump (23) to relieve pressure in the outlet port at a lower than normal pressure level when the fluid flow therethrough is below a predetermined flow rate. Relieving the pressure at the outlet port of the metering pump reduces the pressure differential between the inlet port (24) and outlet port (26) of the metering pump and thereby drastically reduces the internal leakage within the metering pump so that the metering pump is operative to pump fluid from the actuating chamber (14) at the slow rotational speeds of the rotatable member (12).

DESCRIPTION 
1. Technical Field 
This invention relates generally to a brake control system for a spring 
applied, pressure released brake on a winch and more particularly to a 
device for reducing the normal outlet pressure of a metering pump at low 
operating speeds of the winch. 
2. Background Art 
The winches used on pipelayers and the like commonly have a hydraulically 
disengaged brake operative to control the lowering of a supported load by 
controlling rotation of a cable drum of the winch. The brake control 
system sometimes has an accumulator connected between the pump and the 
control valve for providing an auxiliary source of pressurized hydraulic 
fluid for disengaging the brake to lower a suspended load when the pump is 
inoperative. The accumulator is charged to a predetermined pressure 
determined by a relief valve. To achieve steady lowering of the suspended 
load, the fluid pressure in the actuating chamber of the brake must be 
maintained constant. This is sometimes accomplished by a metering pump 
driven by the winch drum to pump fluid from the actuating chamber at a 
rate proportional to the rotational speed of the drum and at a rate 
essentially the same as the rate of flow through the control valve. Thus a 
constant pressure is maintained at a given position of the control valve. 
To conserve fluid when the accumulator is used as the source of fluid 
pressure, and to maintain accumulator pressure in normal operation, the 
metering pump directs the fluid back to the inlet of the control valve in 
a regenerative manner and must thereby pump the fluid against at least the 
pressure established by the relief valve which is substantially higher 
than the pressure required for initial reduction of braking force. 
Some examples of such brake control systems are disclosed in U.S. Pat. No. 
4,048,799 issued to Golan et al. on Sept. 20, 1977 and U.S. Pat. No. 
4,278,155 issued to Golan et al. on July 14, 1981. 
One of the problems associated with such brake control system is that of 
internal leakage between the inlet and outlet ports of the metering pump 
resulting from the pressure differential between the inlet and outlet 
ports. At very slow lowering speeds and relatively high pressures, the 
internal leakage within the metering pump reduces its efficiency so that 
the amount of fluid pumped from the actuating chamber is less than the 
amount of fluid directed to the actuating chamber by the control valve. 
Thus, the fluid pressure in the actuating chamber continues to increase 
for a given setting of the control valve thereby causing greater than 
desired disengagement of the brake and unexpected faster lowering of the 
load. Thus the control system does not provide the precision slow speed 
control required for lowering heavy loads. The internal leakage does not 
cause any problems at moderate or fast lowering speeds wherein the fluid 
flow necessary to maintain constant pressure in the actuating chamber is 
greater than the internal leakage of the metering pump. 
The present invention is directed to overcoming one or more of the problems 
as set forth above. 
DISCLOSURE OF THE INVENTION 
In one aspect of the present invention a brake control system for actuation 
of a spring applied pressure released brake between engaged and disengaged 
conditions for respectively stopping and allowing rotation of a rotatable 
member includes a pump, a control valve having an inlet connected to the 
pump and an outlet connected to an actuating chamber of the brake, said 
control valve being movable between a first position at which the inlet 
port is blocked from the outlet port and a plurality of operative 
positions at which communication between the inlet and outlet ports is 
controllably modulated, a metering pump drivingly connected to the 
rotatable member and having inlet and outlet ports with the inlet port 
being connected to the outlet of the control valve, a conduit connecting 
the outlet port of the metering pump with the inlet port of the control 
valve, means for normally maintaining the pressure in the conduit at a 
predetermined operating level, and means for relieving the pressure in the 
outlet port of the metering pump at a pressure level lower than the 
predetermined operating level when fluid flow therethrough is below a 
predetermined flow rate so that the pressure differential across the 
metering pump is reduced. 
The advantages offered by the present invention is that the metering pump 
is not subjected to the normal back pressure in the outlet port when the 
flow rate therethrough is below a predetermined flow rate. With the fluid 
pressure at the outlet port thus relieved the internal leakage in the 
metering pump is drastically reduced so that the metering pump thereby 
functions in its normal manner of pumping fluid from the actuating chamber 
at a flow rate proportional to the rotational speed of the rotatable 
member thereby allowing precise slow speed lowering of a suspended load.

BEST MODE FOR CARRYING OUT THE INVENTION 
Referring now to the drawing, a brake control system generally indicated by 
the reference numeral 10 provides actuation of a spring applied, pressure 
released brake 11 between engaged and disengaged conditions for 
respectively stopping and allowing rotation of a rotatable member 12. The 
brake 11 typically includes a spring 13 for providing the engaging force 
and an actuating chamber 14 for receiving pressurized fluid to compress 
the spring 13 for disengaging the brake. The rotatable member 12 can be a 
rotatable member of a winch such as a winch drum or a member associated 
with rotation of the winch drum. 
The brake control system 10 includes a pump 16 having its inlet connected 
to a reservoir 17 and its outlet connected to an inlet port 18 of a 
control valve 19 through a conduit 20 and check valve 21. An outlet port 
22 of the control valve 19 is connected to the actuating chamber 14 of the 
brake. The control valve 19 is movable between a neutral position at which 
the inlet port 18 is blocked from the outlet port 22 and a plurality of 
variable operative positions at which communication between the inlet and 
outlet ports is controllably modulated. 
A metering pump 23 has an inlet port 24 and an outlet port 26 and is 
drivingly connected to the rotatable member through a one-way clutch 27. 
The inlet port 24 is connected to the actuating chamber 14 of the brake 
11. The outlet port 26 is connected to the inlet port 18 of the control 
valve 19 through a conduit 28. 
A means 29 is provided for normally maintaining the pressure in the conduit 
28 at a predetermined operating level. The pressure maintaining means 29 
can include a relief valve 30 connected to the conduit 20 between the pump 
16 and the check valve 21, a check valve 31 positioned in the conduit 28, 
and an accumulator 32 connected to the conduit 28 between the check valve 
31 and the inlet port 18. Another relief valve 33 is connected to the 
conduit 28 between the check valve 31 and inlet port 18 to relieve 
excessive pressure in the conduit 28. A conduit 34 connects the relief 
valves 30 and 33 to the reservoir 17. 
While the reservoir 17 is shown symbolically, such reservoir is normally 
the housing containing the winch drive mechanism and the fluid passing 
through conduit 34 is normally directed to the winch drive mechanism for 
lubrication purposes. 
A means 36 is provided for relieving the pressure in the outlet port 26 of 
the metering pump 23 at a pressure level lower than the predetermined 
operating level when fluid flow therethrough is below a predetermined flow 
rate so that the working pressure differential is reduced. Such pressure 
relieving means 36 includes a relief valve 37, a conduit 38 connecting the 
relief valve to the outlet port 26 of the metering pump 23, and a 
restrictive orifice 39 positioned in the conduit 38. A conduit 41 connects 
the relief valve 37 to the conduit 34. 
INDUSTRIAL APPLICABILITY 
In operation, upon initial starting of the pump 16, hydraulic fluid is 
pumped through the conduit 20, the check valve 21 and the conduit 28. With 
check valve 31 blocking fluid flow to the outlet port 26 of the metering 
pump 23, the accumulator 32 is filled to a pressure as determined by the 
relief valve 30 which in the present case is approximately 2600 kPa (375 
psi). After the accumulator is filled, the relief valve 30 opens and 
directs excess fluid through conduit 34 to the reservoir 17. 
To lower a load supported by the rotatable member 12, the control valve 19 
is moved from the neutral position shown to an operative position to 
direct pressurized fluid from the pump 16 to pressurize the actuating 
chamber 14 of the brake 11. This reduces the effective force of the spring 
13 thereby controllably allowing the rotatable member 12 to rotate under 
the influence of the load. The rotational speed of the rotatable member is 
controlled by controlling the fluid pressure in the actuating chamber. The 
pressure in the actuating chamber necessary to release the brake is 
somewhat dependent upon the weight of the load being supported by the 
rotatable member. Rotation of the rotatable member 12 drives the metering 
pump 23 through the one-way clutch 27. The metering pump 23, in turn, 
pumps fluid from the actuating chamber 14 at a flow rate proportional to 
the rotational speed of the rotatable member 12. 
The fluid exiting the metering pump 23 through the outlet port 26 can take 
one of two different flow paths depending upon the flow rate. If the flow 
rate is below a predetermined flow rate such as would occur when the load 
is being inched into place, the fluid passes through the restrictive 
orifice 39, relief valve 37, conduit 41 and empties into the reservoir 17. 
The relief valve 37 is set to open at a pressure lower than the 2600 kPa 
operating pressure normally maintained in the conduit 28. The pressure 
required to open the relief valve 37 is somewhat greater than the fluid 
pressure required to release the brake and in the present invention may be 
somewhere between 900 and 1100 kPa (130 and 160 psi). When the rotatable 
member 12 is rotated at a higher speed, the metering pump 23 pumps fluid 
through the outlet port 26 at a higher flow rate. When the flow rate 
exceeds the capacity of the orifice 39, which in this invention is about 
1.85 liters (0.5 gallons) per minute, sufficient pressure is generated by 
the flow from the metering pump so that such fluid opens the check valve 
31 and a portion thereof passes through conduit 28 back to the inlet port 
18 of the control valve 19 in a regenerative fashion. Under this 
condition, a small portion of the fluid will still pass through the 
orifice 39 and the relief valve 37. 
In view of the foregoing it is readily apparent that the structure of the 
present invention provides an improved brake control system with metering 
pump relief which provides precision control over lowering loads at a slow 
speed. This is accomplished through the use of an orifice and an 
additional relief valve connected in series to the outlet port of the 
metering pump to relieve the pressure in the outlet port at a 
predetermined pressure level in response to fluid flow therethrough being 
below a predetermined flow rate. Thus the pressure differential between 
the inlet port and the outlet port of the metering pump is reduced 
resulting in a drastic reduction in the internal leakage within the 
metering pump at such slow speeds. The metering pump can thus pump fluid 
from the actuating chamber of the brake at a rate proportional to the 
rotational speed of the rotatable member in a normal manner. 
Other aspects, objects and advantages of this invention can be obtained 
from a study of the drawings, the disclosure and the appended claims.