Title: Rotational float for rotating equipment
Abstract: A rotational float system (44) that is meant to be used with existing side load protection systems and that may be selectively operated to permit a digger derrick rotation motor (20) or other rotatable equipment to freely rotate toward a side load to ease operation of and prevent damage to the digger derrick (10) and to assist in the correct installation of screw anchors. The rotational float system (44) includes valve structure (46) interposed between the ports (32, 34) of a hydraulic rotation motor (20) and a control mechanism (48) for selectively opening and closing the valve structure (46). The valve structure (46) is normally biased to a closed position and the control mechanism (48) is normally open. When the control mechanism (48) is operated, it opens the valve structure (46) to connect the ports (32, 34) of the rotation motor (20) to one another. This allows the pressure of the hydraulic fluid present at each of the ports (32, 34) to equalize so that the rotation motor (20) may float or rotate toward any external side loads.
Patent Number: 6,871,710 Issued on 03/29/2005 to Darling, et al.
| Inventors:
|
Darling; W. Brian (Parkville, MO);
Henke; Harlan H. (St. Joseph, MO)
|
| Assignee:
|
Altec Industries, Inc. (Birmingham, AL)
|
| Appl. No.:
|
846979 |
| Filed:
|
May 1, 2001 |
| Current U.S. Class: |
173/184; 173/2; 173/19; 173/42; 173/44 |
| Intern'l Class: |
E21B 007//02 |
| Field of Search: |
173/184,44,43,192,42
|
References Cited [Referenced By]
U.S. Patent Documents
| 3499543 | Mar., 1970 | Updegrave | 212/288.
|
| 3690387 | Sep., 1972 | Dixon | 173/193.
|
| 4093091 | Jun., 1978 | Gregg et al. | 414/642.
|
| 4100843 | Jul., 1978 | Dietrich et al.
| |
| 4100973 | Jul., 1978 | Freudenthal.
| |
| 4106368 | Aug., 1978 | Ivey.
| |
| 4236408 | Dec., 1980 | Corry.
| |
| 4456093 | Jun., 1984 | Finley et al. | 182/2.
|
| 4532595 | Jul., 1985 | Wilhelm | 701/124.
|
| 4752012 | Jun., 1988 | Juergens.
| |
| 5160055 | Nov., 1992 | Gray | 212/278.
|
| 5163570 | Nov., 1992 | Mundis et al.
| |
| 5557526 | Sep., 1996 | Anderson | 701/50.
|
| 6019441 | Feb., 2000 | Lloyd et al.
| |
| 6173573 | Jan., 2001 | Kamada | 60/422.
|
| 6202013 | Mar., 2001 | Anderson et al. | 701/50.
|
| RE37661 | Apr., 2002 | Raunisto | 405/232.
|
Other References
Hoffelmeyer et al, Side Load Detection and Protection System for Rotatable
Equipment, Nov. 7, 2002, US 2002/0165628 A1.
|
Primary Examiner: Smith; Scott A.
Assistant Examiner: Nash; Brian
Attorney, Agent or Firm: Hovey Williams LLP
Claims
Having thus described the preferred embodiment of the invention, what is
claimed as new and desired to be protected by Letters Patent includes the
following:
1. In a rotation motor system including a rotation motor having first and
second ports, first and second media lines respectively operably coupled
with the first and second ports for receipt and discharge of pressurized
media by the motor for operation thereof, and a first, pressure-responsive
control system operably coupled with the motor for protecting the motor
when the motor experiences a pressure exceeding a predetermined level, the
improvement, which comprises a second, pressure-independent control
system, comprising:
a valve interposed between the first and second media lines and shiftable
between a closed position wherein the first and second media lines are
substantially isolated from one another and a pressure differential exists
between the first and second media lines, and an open position wherein the
first and second media lines are in communication with one another and the
pressure differential between the first and second lines is reduced as
compared with the pressure differential when the valve is in the closed
position; and
a manually controlled control mechanism coupled with the valve and operable
to shift the valve from the closed to the open position independently of
the pressure-responsive operation of the first control system and of the
pressure differential between the first and second media lines.
2. The rotation motor system as set forth in claim 1, the valve comprising
a pair of poppet-type valves in fluid communication with one another
between the first and second media lines.
3. The rotation motor system as set forth in claim 1, the valve comprising
a blocking valve interposed between the first and second media lines.
4. The rotation motor system as set forth in claim 1, the control mechanism
comprising a switch for electrically controlling switching of the valve
between the closed and the open positions.
5. The rotation motor system as set forth in claim 1, the pressurized media
comprising hydraulic fluid.
6. The rotation motor system as set forth in claim 1, further including a
tilt switch operatively coupled with the control mechanism for preventing
the control mechanism from switching the valve to the open position.
7. The rotation motor system as set forth in claim 6, further including an
indicator operatively coupled with the control mechanism for indicating
when the control mechanism has switched the valve to the open position.
8. The rotation motor system as set forth in claim 7, the indicator
including an audible alarm or a visible alarm.
9. The rotation motor system as set forth in claim 1, further including an
electrical relay interposed between the control mechanism and the valve.
10. The rotation motor system as set forth in claim 1, further including a
flow control device positioned in-line with the valve for controlling a
rate at which the valve allows equalization of pressure between the first
and second ports of the rotation motor when the valve is switched to the
open position.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to rotatable equipment such as digger
derricks. In particular, the invention relates to a rotational float
system that may be selectively operated to permit a digger derrick boom to
freely rotate toward a side load to simplify operation of and prevent
damage to the digger derrick.
2. Description of the Prior Art
Digger derricks, cranes, and other types of rotating equipment are commonly
used to perform many different jobs, including digging holes for utility
poles and installing screw anchors for supporting the poles. Digger
derricks are typically mounted to mobile utility vehicles and include a
rotating turntable from which a boom extends. The boom may be raised,
lowered, extended, and retracted to position its outboard end in various
locations to perform digging or hoisting operations. Digger and auger
assemblies may be coupled with the outer end of the boom for digging holes
for utility poles and for installing screw anchors. Once installed,
anchors are typically connected to guy wires to stabilize the top portion
of a pole in a particular direction and therefore are preferably installed
into the ground at some angle relative to vertical.
Digger derricks and other rotating equipment are often subjected to
excessive and undesirable side loading. Side loading can be applied to a
digger derrick boom when, for example, a winch attached to the boom is
used to pull a heavy load in from the side of the boom. Side loading also
often occurs when an auger-type digger coupled with the boom "corkscrews"
into the ground due to the application of excessive pressure in driving
the auger or when anchors are improperly installed. Excessive side loading
can cause damage or destruction of the digger derrick's boom, turntable,
and rotation drive mechanism. Improper installation of an anchor can also
significantly reduce its pull-out strength.
Special care must be taken when installing screw anchors to avoid or
minimize such side loading. When a screw anchor is to be installed
directly behind a truck to which a digger derrick is mounted (so that the
truck is in-line with the direction of the anchor), it is necessary for
the digger operator to monitor and control at least three functions to
ensure that the boom generally follows the path of the anchor.
Specifically, the operator must simultaneously operate the digger motor,
lower the boom, and retract the boom as the anchor is being driven into
the ground. If an anchor is to be installed on either side of the truck,
as is common with the majority of roadside pole installations, it is also
necessary for the digger operator to rotate the turntable and the boom
toward the anchor as the anchor enters the ground to correctly follow the
path of the anchor. Those familiar with the operation of digger derricks
will appreciate that simultaneously monitoring all of these movements
requires a great deal of skill and training. If an operator forgets or
otherwise fails to correctly follow the path of an anchor into the ground
by manipulating the digging, boom extending/retracting, boom
lowering/raising, and rotating functions, the anchor will tend to pull the
boom to one side, exerting considerable side loading on the derrick and/or
reducing the pull-out strength of the anchor.
Systems have been developed for protecting digger derricks and other
rotating equipment against excessive side loads. U.S. Pat. No. 4,100,973
(the '973 patent), hereby incorporated into the present application by
reference, discloses a side load protection system that attempts to sense
undesired side load levels and respond with appropriate action when
pre-established limits have been exceeded. The system uses relief valves
that open when pressure increases beyond a desired level in a hydraulic
motor to allow the boom driven by the motor to slip or rotate toward the
external load, thereby reducing the torque of the side load. Other side
load protection systems are being developed that shut down certain digger
derrick operations once side load limits have been reached.
Although generally effective, existing side load protection systems are
subject to misuse. For example, operators who are aware of side load
protection systems may intentionally neglect to rotate a digger derrick's
boom to follow an anchor into the ground, knowing that the side load
protection system will permit the turntable and boom to slip or rotate
toward the side load once an excessive amount of side load has been
exerted on the boom. This practice should be discouraged because existing
side load protection systems require application of a high degree of side
loading before they are activated. Such side load protection systems
therefore are intended to provide protection from occasional high side
loads, not repeated side loads resulting from everyday operation such as
the installation of screw anchors. If an operator routinely relies on a
side load protection system during normal operation of a digger derrick,
the cumulative effect of the side loading that occurs before the side load
protection system is engaged can significantly damage the digger derrick
and reduce the holding power of any installed screw anchors.
SUMMARY OF THE INVENTION
The present invention solves the above-described problems and provides a
distinct advance in the art of side load protection systems for digger
derricks and other rotating equipment. More particularly, the present
invention provides a rotational float system that is designed for use with
existing side load protection systems and that may be selectively operated
to permit a digger derrick rotation motor or other rotatable equipment to
freely rotate toward a side load to ease operation of and prevent damage
to the digger derrick.
The rotational float system of the present invention broadly includes valve
structure interposed between the ports of a hydraulic rotation motor and a
control mechanism for selectively opening and closing the valve structure.
The valve structure is normally biased to a closed position and the
control mechanism is normally open. When the control mechanism is
operated, it opens the valve structure to connect the ports of the
rotation motor to one another. This allows the pressure of the hydraulic
fluid present at each of the ports to equalize so that the rotation motor
may float or rotate toward any external side loads.
The rotational float system of the present invention may be selectively
activated by an operator during normal operations of a digger derrick such
as when a screw anchor is being installed. Once the bite of an anchor has
been established, the operator may simply activate the control mechanism
to open the valve structure and place the rotation motor in a float state.
As the anchor travels further into the ground, it will exert a side load
on the boom that will create a pressure differential between the two ports
of the rotation motor. When activated, the rotational float system allows
this pressure differential to equalize, causing the boom and turntable to
rotate toward the anchor. This alleviates the side load and thus relieves
the operator from the necessity of monitoring and controlling the rotation
of the boom and turntable so the operator may concentrate on monitoring
other necessary functions of the digger derrick.
These and other important aspects of the present invention are described
more fully in the detailed description below.
BRIEF DESCRIPTION OF THE DRAWING FIGURES
A preferred embodiment of the present invention is described in detail
below with reference to the attached drawing figures, wherein:
FIG. 1 is a schematic top plan view illustrating a conventional digger
derrick which may be equipped with the invention described herein and
which is shown with its boom positioned in-line with the truck to which it
is mounted;
FIG. 2 is a schematic top plan view of the digger derrick shown with its
boom positioned to the side of the truck to which it is mounted; and
FIG. 3 is a schematic diagram of a rotational float system constructed in
accordance with a preferred embodiment of the present invention shown
coupled with a portion of a hydraulic system of the digger derrick
illustrated in FIGS. 1 and 2.
The drawing figures do not limit the present invention to the specific
embodiments disclosed and described herein. The drawings are not
necessarily to scale; emphasis instead being placed upon clearly
illustrating the principles of the invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Turning now to the drawing figures, and particularly FIGS. 1 and 2, a
conventional truck-mounted digger derrick 10 of the type with which the
present invention is adapted for use is illustrated. The digger derrick 10
broadly includes a frame 12 mounted on a truck bed 14 that rotatably
supports a turntable 16. An elongated boom 18 extends outwardly from the
turntable 16 and is operable to rotate with the turntable 16.
The turntable 16 and the boom 18 are rotatably driven by a hydraulic
rotation motor 20 (FIG. 3) mounted to the turntable 16. The rotation motor
20 delivers rotational force to the turntable 16 through conventional
drive linkage. The boom 18 may be pivoted up and down relative to the
turntable 16 by a conventional hydraulic cylinder system and includes a
plurality of sections 18a, 18b, 18c that may extend and retract relative
to one another in a telescopic fashion to vary the overall length of the
boom 18. A hydraulically-powered winch 22 may be coupled with the
turntable 16 and/or the boom 18.
When the digger derrick 10 is to be used to dig holes for utility poles or
to install earth or screw anchors for supporting the poles, a driving tool
24 may be mounted to the intermediate section 18b of the boom. The driving
tool 24 includes a digger power unit 26 that rotates a shaft 28 for
turning a conventional earth auger (not shown) or a screw anchor 30.
FIG. 3 illustrates schematically portions of the hydraulic system which
controls operation of the rotation motor 20. The hydraulics for operating
other portions of the digger derrick 10, including the systems for
raising, lowering, extending and retracting the boom 18 are eliminated
from the schematic of FIG. 3 for purposes of simplicity.
As illustrated, the rotation motor 20 includes a pair of first and second
hydraulic ports 32, 34 for receiving and discharging a pressurized media
such as hydraulic fluid or oil from conventional hydraulic lines 36, 38
that receive hydraulic fluid from a source thereof. The digger derrick 10
is also preferably equipped with a hydraulically-released brake 40 that,
when engaged, prevents the rotation motor 20 and drive linkage coupled
thereto from being driven either forwardly or backwardly to prevent
rotational movement of the turntable 16 and the boom 18. The brake 40 is
automatically released whenever the rotation motor 20 and/or digger power
unit 26 is operated. Thus, when the boom 18 is being rotated and/or when
the digger power unit 26 is being operated, any side loads applied to the
boom 18 are held hydraulically by the rotation motor 20. The
hydraulically-operated brake 40 and its operation are described in more
detail in the '973 patent referenced above.
The digger derrick 10 is also preferably equipped with a side load
protection system 42 that reduces or prevents damage to the boom 18 and
other portions of the digger derrick 10 when an undesired side load is
applied to the boom 18. One such side load protection system is disclosed
in the '973 patent referenced above and another is disclosed in pending
patent application entitled "A Side Load Detection and Protection System
for Rotatable Equipment", Ser. No. 09/846,977, filed May 1, 2001, which is
now U.S. Pat. No. 6,735,486 entitled "Side Load Detection and Protection
System for Rotatable Equipment", also hereby incorporated into the present
application by reference.
As discussed previously, the side load protection system 42 is intended to
provide protection from occasional high side loads, but should not be
relied upon to relieve repeated side loads resulting from every day
installation of screw anchors 30. If an operator routinely relies on the
side load protection system 42 when installing screw anchors 30 and
performing other normal functions, the cumulative effect of the side
loading that occurs before the side load protection system 42 is engaged
can significantly damage the digger derrick 10 over time and reduce the
holding power of installed screw anchors 30.
The present invention provides a rotational float system broadly referred
to by the numeral 44 in FIG. 3 which is designed to complement the side
load protection system 42 by providing relief from side loads caused by
everyday operation of the digger derrick 10. FIG. 3 illustrates a
preferred embodiment of the rotational float system 44, which broadly
includes valve structure 46 interposed in the hydraulics of the rotation
motor 20 and a control mechanism 48 operatively coupled with the valve
structure 46 for controlling operation thereof.
The valve structure 46 is preferably interposed between the hydraulic lines
36, 38 connected to the first and second ports 32, 34 of the rotation
motor 20 but can be positioned anywhere as long as it is in fluid
communication with both ports 32, 34 of the rotation motor 20. The valve
structure 46 is normally closed so that the hydraulic lines 36, 38 are
substantially isolated from one another and can be selectively switched to
an open position to place the hydraulic lines 36, 38 in fluid
communication with one another. When the valve structure 46 is switched to
this open position, hydraulic fluid may flow freely between the two
hydraulic lines 36, 38, thus equalizing the hydraulic pressure between the
first and second ports 32, 34 of the rotation motor 20. This places the
rotation motor 20 in a "float" condition or state and enables the
turntable 16 and boom 18 driven by the rotation motor 20 to freely float
or rotate toward a side load.
The valve structure 46 preferably comprises a pair of back-to-back
poppet-type solenoid valves 50, 52 connected to the hydraulic lines 36, 38
by conventional hydraulic circuitry 54. The two poppet-type solenoid
valves 50, 52 may be replaced with any type of blocking valve that
prevents flow through line 54, but bidirectional blocking is best achieved
by using the preferred arrangement of two back-to-back poppet-type
solenoid valves 50, 52 as illustrated.
The valve structure 46 also preferably includes an orifice or other flow
control 56 that regulates the rate at which hydraulic fluid may flow
therethrough to control the rate at which pressure is equalized between
the two ports 32, 34 of the rotation motor 20. The orifice 56 may be
positioned anywhere in the hydraulic circuitry 54, for example, between
the two solenoid valves 50, 52. The orifice 56 slows the rotation speed of
the turntable 16 and boom 18 to an acceptable level when the valve
structure 46 is switched to its open position and the boom 18 is subjected
to a side load. Those skilled in the operation of digger derricks will
appreciate that regulating the rotation speed of the turntable 16 and boom
18 is necessary, especially when the digger derrick 10 is on a slope, to
prevent unexpected boom movement. The size of the orifice 56 may be
selected to allow any rate of hydraulic fluid flow therethrough and may
even be replaced with a variable-type flow mechanism so that the rotation
speed of the turntable 16 and boom 18 may be selectively adjusted.
The control mechanism 48 is electrically connected between the valve
structure 46 and a terminal 58 on a digger control panel 60 positioned
within reach of an operator of the digger derrick 10. The control
mechanism 48 allows an operator to selectively switch the valve structure
46 from its normally closed position to an open position to place the
rotation motor 20 in a float state. The terminal 58 is only powered when a
control lever or joystick 62 on the digger control panel 60 is displaced
from its center position to power the digger power unit 26. This prevents
the rotational float system 44 from placing the rotation motor 20 in a
float state unless the digger power unit 26 is operated.
The control mechanism 48 is preferably electrically isolated from the valve
structure 46 by a relay 64. The relay 64 permits the control mechanism 48
to be powered by a low-amp signaling line and the valve structure 46 to be
powered by a higher-amp line.
The control mechanism 48 may be any type of switch or controller, but is
preferably a push-button switch positioned within reach of the digger
derrick's digger control lever or joystick 62. The control mechanism 48
may be, for example, a push button switch located on the tip of the digger
joystick 62 so that it can be easily actuated by an operator's thumb while
the operator manipulates the joystick 62 to control the digging operation
of the derrick 10. The control mechanism 48 is preferably normally open,
momentarily closed so that it must be held in its closed position to
switch the valve structure 46 to its open, float state. As soon as the
control mechanism 48 is released, it opens and thereby causes the valve
structure 46 to close.
The rotational float system 44 also preferably includes a tilt switch 66
connected in series with the control mechanism 48. The tilt switch 66 is
normally closed and opens to disable the control mechanism 48 whenever the
digger derrick 10 is positioned on a slope equal to or exceeding a
predetermined amount. For example, the tilt switch 66 may be set to open
whenever the digger derrick 10 is on a slope of 5.degree. or more. This
prevents the operator from placing the rotation motor 20 in a float
condition whenever the digger derrick 10 is on a slope for safety
purposes. The tilt switch 66 is preferably an omni-directional,
pendulum-type switch. The slope threshold for the switch may be selected
to complement the size of the orifice 56 so as to obtain an acceptable
boom rotation speed for a given slope threshold. For example, if the
orifice 56 is sized to provide a very slow boom 18 and turntable 16
rotation speed, the slope threshold of the tilt switch 66 may be greater,
but if the orifice 56 is sized to allow faster rotation of the boom 18 and
turntable 16, the slope threshold of the tilt switch 66 should be lower.
The rotational float system 44 also preferably includes an alarm or
notification system for alerting the operator and any other nearby persons
when the boom 18 and turntable 16 are placed into a float state. The alarm
system preferably includes an audible alarm 68 and a visible alarm 70 that
may be on or near the digger derrick 10 and that are both activated
whenever the rotational float system 44 is enabled.
Operation
The digger derrick 10 may be operated in a conventional manner to perform
various hoisting and digging operations such as digging holes for utility
poles and installing screw anchors 30 for supporting utility poles. During
normal operation of the digger derrick 10, the boom 18 may be subjected to
undesirable side loading. The rotational float system 44 may be activated
when operating the dig function to place the rotation motor 20 and the
boom 18 and turntable 16 coupled therewith in a rotational float state to
rotate toward and relieve such side loading.
For example, FIG. 1 illustrates the installation of a screw anchor 30
directly behind, or in-line, with the digger derrick 10. For these types
of screw anchor 30 installations, the operator must simultaneously monitor
and control the operations of the digging power unit 26, the hydraulic
systems controlling lowering and raising of the boom 18, and the hydraulic
systems controlling extending and retracting of the boom 18 as the screw
anchor 30 bites into the ground so as to follow the path of the screw
anchor 30. If the operator fails to do so, the pull-out strength of the
anchor 30 may be compromised and the boom and the driving tool may be
damaged. However, for installation applications as illustrated in FIG. 1,
the operator does not have to monitor and control rotation of the boom 18
and turntable 16 because the anchor 30 is being installed behind, or
in-line with the digger derrick 10. Thus, no rotational side loads are
placed on the turntable 16 and boom 18 as a result of the anchor 30
entering the ground.
FIG. 2 illustrates the more common installation of a screw anchor 30 to the
side of the digger derrick 10. This type of installation is more common
because utility poles and anchors 30 for supporting the poles are commonly
installed along the sides of roads, and the digger derrick 10 cannot be
positioned directly in-line with the hole to be dug or the anchor 30 to be
installed. For these types of screw anchor 30 installations, the operator
must simultaneously monitor and control all of the operations described in
the previous paragraph and additionally must rotate the turntable 16 and
boom 18 toward the anchor 30 as the anchor 30 is driven into the ground so
as to follow the path of the anchor 30. If the operator fails to do so,
the anchor 30 will tend to pull the boom 18 toward it, exerting
considerable side loading on the digger derrick 10.
Side loading will create a differential pressure between the two ports 32,
34 of the rotation motor 20. While the digger is being operated, the brake
40 on the rotation motor 20 will be released, causing any rotational side
load to be held hydraulically by the rotation motor 20. If an excessive
side load that equals or exceeds the threshold of the side load protection
system 42 is developed, the side load protection system 42 will either
alleviate the side load by allowing the boom 18 and turntable 16 to rotate
toward the load or will disable all functions of the digger derrick 10
that could be causing the side load. However, as described above, by the
time the side load protection system 42 is enabled, the side load may have
already caused damaged to the digger derrick 10 and/or reduced the pullout
strength of an installed anchor 30, especially if the side load protection
system 42 is repeatedly enabled during routine operation of the digger
derrick 10.
The rotational float system 44 of the present invention allows an operator
of the digger derrick 10 to selectively relieve side loads caused by
normal operation of the digger derrick 10 before the side load protection
system 42 is activated. For example, the rotational float system 44 may be
selectively activated by an operator any time a screw anchor 30 is being
installed. Once the bite of an anchor 30 has been established, the
operator may simply activate the control mechanism 48 to open the valve
structure 46 and place the rotation motor 20 in a float state. As the
anchor 30 travels further into the ground, it will exert a side load on
the boom 18 that will attempt to create a pressure differential between
the two ports 32, 34 of the rotation motor 20. The activated rotational
float system 44 allows this pressure differential to equalize, causing the
boom 18 and turntable 16 to rotate toward the anchor 30. This alleviates
the side load, relieving the operator from the necessity of monitoring and
controlling the rotation of the boom 18 and turntable 16 and thus allowing
the operator to concentrate on other necessary functions of the digger
derrick 10.
In another embodiment of the present invention, the above-described "float"
condition can be achieved using existing holding valves for the rotation
motor 20. Holding valves are used to prevent the rotation motor 20 from
free-wheeling if hydraulic fluid lines leading to the motor break.
Typically, one holding valve is interposed in the media line 36 connected
to the first port 32 of the rotation motor 20 and a second holding valve
is coupled with the media line 38 connected to the second port 34 of the
rotation motor 20. The holding valves in turn are connected to a
directional valve that serves as the main control valve for controlling
rotation of the motor 20. The center position of this directional valve
connects the fluid lines leading to the holding valves. Thus, when the
directional valve is in its center position, one or both of the holding
valves may be opened to equalize the pressure between the first and second
media lines 36, 38 and the pressure between the first and second ports 32,
34 of the rotation motor 20.
Although the invention has been described with reference to the preferred
embodiment illustrated in the attached drawing figures, it is noted that
equivalents may be employed and substitutions made herein without
departing from the scope of the invention as recited in the claims. For
example, although the present invention is especially useful when equipped
with a digger derrick 10, it may be used in connection with any machine or
system that utilizes a rotation motor and that is subject to undesirable
side loads.
*
