Mobile crane

A straddle type mobile crane includes a steering system which allows two-wheel or four-wheel steering. A first steering cylinder is associated with each wheel for four-wheel operation and an additional cylinder is associated with each of a first pair of wheels for two-wheel steering. An interlock system has a first mode for four-wheel drive wherein each wheel is coupled to its associated first steering cylinder and a second mode wherein each wheel is uncoupled from each first cylinder and each of the first pair of wheels are coupled to one of the additional cylinders and the other pair of wheels are locked to the frame. A first control prevents operation of the interlock unless the wheels are in predetermined orientation and a second control maintains proper alignment of the wheels in all angular positions.

BACKGROUND OF THE INVENTION 
This invention relates to a mobile type gantry crane. 
Straddle type gantry cranes are commonly employed for lifting and moving 
large bulky objects, such as shipping containers, structural members, 
modular building sections, heavy machinery and equipment and the like. 
Such devices are well known and generally include a pair of inverted 
U-shaped gantries comprising cross beams spanning vertical columns which 
are supported at their lower ends on wheels or truck assemblies. The two 
gantry assemblies are interconnected by spaced side members and may 
include hoists supported from the cross beams by means of a trolley. This 
permits the load to be elevated and traversed laterally. 
Mobile type cranes must be sufficiently maneuverable for being positioned 
above the load prior to elevation and for transporting the load from one 
location to another. When operated in a shop or storage area, for example, 
the crane will normally move longitudinally up and down an aisle and in 
addition, laterally between aisles. This requires a relatively flexible 
steering assembly. In addition, because of the large weight involved, 
particularly when a load is being transported, improper wheel orientation 
can cause inordinate stresses. 
SUMMARY OF THE INVENTION 
It is an object of the invention to provide a new and improved steering 
system for a gantry type crane. 
A further object of the invention is to provide a steering system for 
gantry type cranes capable of both two and four wheel steering. 
Another object of the invention is to provide a steering system for gantry 
type cranes in which proper wheel orientation is maintained. 
These and other objects and advantages of the present invention will become 
more apparent from the detailed description thereof taken with the 
accompanying drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to FIGS. 1 and 2, the gantry type mobile crane to which the 
present invention is applicable is shown to include a pair of U-shaped 
gantries 10 and 11 connected by spaced side girders 12. Each gantry 
includes a pair of vertical column members 13 supported at their lower 
ends by truck assemblies 14 and at their upper ends by horizontal cross 
members 16. For purposes of discussion, the reference numeral 14 will be 
used when discussing the trucks 14 collectively while the numerals and 
letters 14RF, 14LF, 14RR and 14LR will be used to specifically identify 
the trucks located at the right front, left front, right rear and left 
rear respectively of the assembly. Other portions of the assembly 
discussed below will similarly be identified depending upon whether the 
portions are discussed generally or specifically. 
Each gantry 10 and 11 also includes a lifting assembly 18 which are 
identical and accordingly only one will be described for the sake of 
brevity. Each lifting assembly 18 includes a winch 20, a trolley 22, idler 
sheaves 23, 24 and 25 and a hook block 27. As those skilled in the art 
will appreciate, the trolley 22 includes wheels (not shown) which ride on 
the flanges 28 on each of the opposite sides of its associated cross beams 
16. A wire rope 30 extends upwardly from each winch 20, over sheave 23, 
around a first sheave 32 on trolley 22, downwardly and around a first 
pulley 34 on hook block 27, upwardly and over a second sheave 36 on 
trolley 22, downwardly and around a second pulley 38 on block 27, upwardly 
and over a third sheave 40 on trolley 22, around sheave 25 at the opposite 
end of cross beam 16, backwardly along the opposite side of cross beam 16, 
around additional sheaves 40, 36 and 32 and pulleys 38 and 34 on the 
opposite sides of trolley 22 and block 27, around sheave 24 and back to 
the winch 20. Because the operation of the trolley 22 and hook block 27 
form no part of the invention, they will not be discussed in detail. It 
will be sufficient for purposes of understanding the invention merely to 
state that the trolley may be moved longitudinally on the cross beams 16 
and the hook block may be elevated for the purpose of positioning, lifting 
and replacing the load. 
Each truck 14 includes a yoke 42 upon which an axle 44 rotatably supports a 
pair of wheels 46. The yokes 42 are pivotally mounted for movement about 
vertical axes by means of a vertically extending king pin 48 suitably 
received in a bearing (not shown) in the lower end of its associated 
column 13. Also mounted in each yoke 42 is a hydraulic drive motor (not 
shown) for rotating wheels 46 whereby the assembly may be moved from one 
location to another. 
Each front truck assembly 14RF and 14LF is provided with a first steering 
assembly 50R and 50L, respectively, which are employed for two wheel 
steering and a second steering assembly 52R and 52L, respectively, which 
are employed during four wheel steering. In addition, each rear truck 
assembly 14RR and 14LR is respectively provided with a steering assembly 
54R and 54L which is also employed during four wheel steering. It will be 
appreciated, therefore, that during two wheel steering, only the front 
trucks 14RF and 14LF will be turned while the rear trucks 14RR and 14LR 
will be locked in a position generally parallel to the side girders 12. 
During four wheel steering, however, all four trucks 14 will be coupled to 
its associated four wheel steering assembly 52 or 54. 
The steering assemblies 50R and 52R for the truck 14RF are shown more 
particularly in FIGS. 3 and 4. Steering assembly 50R includes a first 
cylinder 56R pivotally mounted at one end by means of a vertically 
extending pin 58 and a bracket 60 affixed to the side of girder 12. A 
piston rod 62 extends from the other end of cylinder 56 and is pivotally 
connected at its end by means of a pin 63 to a bell crank 64 at a point 
intermediate its ends. One end of bell crank 64 is pivotally connected by 
means of a vertically extending pin 66 to a bracket 68 mounted on the 
other side of girder 12. The free end of bell crank 64 is pivotally 
connected by means of a pin 70 to one end of a link 72 which extends 
transversely below girder 12 where its opposite end is pivotally connected 
by pin 74 to one corner of a generally rectangular crank arm 76. An 
enlarged opening 78 (FIG. 4) is formed at a second corner of crank arm 76 
for being received on the king pin 48RF for pivotal movement therearound. 
The other corner of arm 76 extends over the front of yoke 42RF and has a 
smaller aperture 80 for receiving a shot pin 82 which is movable into and 
out of opening 80 by means of a shot pin cylinder 84R (FIG. 3) mounted on 
the front surface of yoke 42. 
FIG. 3 shows the steering assembly 50R in a neutral position wherein the 
wheels 46 are pointed forwardly and the piston rod 62 is in a midpoint 
position relative to cylinder 56R. When it is desired to turn the wheels 
in a counter-clockwise direction as viewed in FIG. 3, the piston rod 62 is 
forced outwardly of the cylinder 56R thereby rocking bell crank 64 
clockwise and moving links 72 laterally toward crank arm 76. This pivots 
crank arm 76 counter-clockwise and also turns the yoke 42RF in the same 
direction as a result of the coupling therebetween by shot pin 82. 
Movement of the wheels in a clockwise direction is accomplished by moving 
piston rod 62 inwardly of cylinder 56R thereby rocking bell crank 64 
counterclockwise to move the link 72 in a direction away from crank arm 76 
so that crank arm 76 and yoke 42RF will turn in the clockwise direction. 
The four wheel steering assembly 52R includes a second cylinder 86R 
pivotally connected by a pin 88 extending vertically through bracket 60 at 
a point slightly above and to the side of pin 58. Piston rod 90 extending 
from cylinder 86 is pivotally connected by means of a pin 91 to the end of 
a crank arm 92. The other end of arm 92 has an enlarged aperture 94 which 
is coradial with king pin 48RF and is pivotally received thereon at a 
location above crank arm 76 and spaced therefrom by a spacer 96. Arm 92 
extends in a generally forward direction from king pin 48RF and has an 
aperture 98 formed intermediate its ends for receiving a shot pin 100 
which is movable into and out of aperture 98 by means of a shot pin 
cylinder 102R mounted on the front of the fork 42RF and generally adjacent 
shot pin cylinder 84R. 
When it is desired to effect four wheel steering, the shot pin 82 is moved 
out of aperture 80 so that crank arm 76 is uncoupled from yoke 42RF and 
shot pin 100 is moved into aperture 98 to couple arm 92 to yoke 42RF. 
Piston rod 90 is in an extended position relative to piston 86R when the 
wheels 46 are oriented in a forward direction. As a result, movement of 
piston rod 90 into cylinder 86 will pivot arm 92 and yoke 42RF 
counterclockwise as viewed in FIG. 3 through an angle of up to about 
95.degree.. Return movement of piston rod 90 out of cylinder 86 will, of 
course, move yoke 42RF in the counterclockwise direction until the wheels 
46 are reoriented in their forward direction. 
FIG. 5 shows the third steering assembly 54R from below as applied to the 
right side of the assembly. The assembly 54R includes a cylinder 104R 
pivotally mounted at one end by a vertically extending pin 106 through a 
bracket 108 affixed to the right side girder 12. Extending from cylinder 
104 is a piston rod 110 whose distal end is pivotally coupled by pin 112 
to the end of an arm 114 connected to and extending laterally from the 
rear of yoke 42RR. Disposed above the arm 114 and affixed to the column 13 
is a shot pin cylinder 118R operative to move a shot pin 120 into and out 
of an aperture 122 in arm 114. 
When the gantry is to be operated in the two wheel mode, the shot pin 120 
is disposed within aperture 122 to lock yoke 42RR to column 13 with the 
wheels 46 pointed forwardly. For four wheel operation, the shot pin 120 is 
withdrawn so that the cylinder 104R can pivot the yoke 42RR. As was the 
case with cylinder 86R, the cylinder rod 110 is in its fully extended 
position when the yoke 42RR is directed forwardly. Therefore, the yoke 
42RR can only be pivoted between its position shown in FIG. 5 and a second 
position about 95.degree. in the counterclockwise direction. 
It will be appreciated that the front forks 42RF and 42LF and the rear 
forks 42RL and 42RR must be in a proper alignment in order to move the 
shot pins 82, 100 and 120 into their respective openings 80, 98 and 122. 
Toward this end, a position control assembly 124 is provided at each fork. 
One such assembly 124RF in association with a front fork 42RF is shown in 
FIG. 6. Specifically, the assembly 124RF includes a photocell 126 
supported on a bracket 128 which is in turn affixed to the yoke 42RF for 
pivotal movement therewith about the axis of king pin 48RF. Extending 
upwardly from bracket 128 is a generally L-Shaped support 130 for 
positioning a light source 132 above and in alignment with photocell 126. 
In addition, an arcuate shield 136 is supported from the base of column 13 
by bracket 134 between photocell 126 and light source 132. The center of 
curvature of shield 136 lies along the axis of king pin 48RF and the radii 
of curvature of its inner and outer edges straddle the photocell 126 and 
light source 132. In addition, the arcuate extent of shield 136 is greater 
than the turning angle of the yoke 42RF. Accordingly, the shield 136 will 
be fixed relative to the photocell 126 and light source 132 and will at 
all times be disposed therebetween. However, shield 136 does have a small 
aperture 138 formed therein and positioned to lie in registry with 
photocell 126 and light source 132 when the wheels 46 are oriented 
forwardly and the shot pins 82 and 100 are in registry with their 
respective apertures 80 and 98. As will be discussed in greater detail 
below, the photocell 126 is coupled to the activating system for the shot 
pin cylinders 84 and 102 so that their operation is prevented except when 
yoke 42F is in a proper angular position. While the preferred embodiment 
is illustrated in connection with photo-switches, conventional limit 
switches may also be employed to indicate angular alignment. 
The position control assembly 124 also includes a potentiometer 138RF 
supported by bracket 128 along the axis of fork 42RF. The wiper 140RF of 
potentiometer 138RF is affixed to column 13 and remains stationary as 
potentiometer 138RF rotates along the axis of fork 42RF. As a result, the 
voltage across potentiometer 138RF will vary with the angular position of 
fork 42RF to provide an angle position indication for purposes which will 
be disclosed below. 
While the steering assemblers 50R, 52R and 54R have been discussed with 
respect to one side of the assembly, it will be appreciated that there are 
identical steering assemblies at the apposite side which are mirror images 
of those described. It will be appreciated that a position control 
assembly identical to 124RF will be provided at each fork so that 
operation of the associated shot pin cylinders can occur only when the 
forks are in proper alignment and further so that an electrical indication 
will be provided with respect to the angular position of each fork. When 
specific reference is made to those portions of the apparatus which have 
not heretofore been described but which are located at other locations of 
the apparatus, corresponding parts will be given the same numerals as 
those described above but they will be distinguished by the letters R, L, 
RF, LF, RR or LR depending respectively whether the assembly is disposed 
at the right, left, right front, left front, right rear or left rear 
portions of the apparatus. On the other hand when like parts of these 
assemblies at different locations of the apparatus are referred to 
collectively, only the reference numeral will be used without the letter 
designations. 
Reference is now made to FIG. 7 which schematically illustrates a portion 
of the hydraulic and electrical circuits for selectively coupling the 
various steering and shot pin cylinders to a pump 140. Specifically, a 
first valve 142 is operable to selectively couple pump 140 to the shot pin 
cylinders 84, 102 or 118 for switching between four-wheel and two-wheel 
steering. Additionally, second and third valves 144 and 145 are operative 
to selectively couple steering cylinders 56, 86 and 104 at each side of 
the assembly, respectively, to pump 140. The valves 142, 144 and 145 are 
four-way, spring centered, directional valves each operable by solenoids 
146 and 147. 
FIG. 7 shows the system in the two-wheel steering mode wherein the 
solenoids 146 of valves 144 and 145 are energized and solenoid 146 of 
valve 142 is de-energized. Those portions of the circuitry not necessary 
for understanding of the operation of the apparatus in this mode have been 
omitted but can be seen in FIG. 9. When in this mode, valve 142 is in a 
neutral position so that pump 140 is connected through valve 142 to an 
orbital steering valve 148 through conduits 152 and 154. Also, valve 144 
connects two-wheel steering cylinder 56L to the steering valve 148 through 
conduits 156 and 157 and valve 145 connects two-wheel steering cylinder 
56R to valve 148 through conduits 158 and 160. Steering valve 148 may be 
manipulated by an operator when turning a steering wheel 162 to 
selectively connect valves 144 and 145 to the pump 140 or to a sump 164 
through conduit 166. In addition, the opposite ends of cylinders 56L and 
56R are connected by a conduit 167 so that the cylinders will move in 
opposite directions and through the same displacement, depending upon the 
positions of the steering valve 148 as set by the operator. 
Respectively associated with each of the shot pin cylinders 84, 102 and 118 
are first limit switches 168, 169 and 170. The first limit switches in 
turn control contacts which are in the energizing circuits of the 
solenoids 146 of valves 144 and 145. Specifically, solenoids 146 of the 
valves 144 and 145 are coupled to a battery 173 by a circuit consisting of 
conductors 174, 175, 176, 177, 206 and the closed contacts 178, 180 and 
182 of limit switches 168, 169 and 170. It can be seen, therefore, that in 
order for solenoids 146 of valves 144 and 145 to remain energized and 
thereby couple steering cylinders 56 to pump 140, each of the contacts 
178, 180 and 182 must be closed. This condition will prevail as long as 
shot pin cylinders 84 and 118 are in their fully extended positions and 
shot pin cylinders 102 are in their fully retracted positions. This 
insures that hydraulic fluid under pressure will not be delivered to 
steering cylinders 56 for two-wheel steering unless the shot pin cylinders 
84, 102 and 118 are in the correct position for the two-wheel steering 
mode. 
In order to switch the system from the two-wheel to the four wheel steering 
mode, the wheels must first be brought to the zero deflection position. 
This will close each of the photo switches 126 as discussed with respect 
to FIG. 6. A manual switch 184 may then be closed so as to energize a 
relay 186 through conductor 174, contacts 126, conductor 188, contacts 190 
of relay 192, conductor 194, and conductor 176. As a result, the contacts 
200 of relay 186 are stepped from their positions by full lines to that 
shown by broken lines. This completes an energizing circuit between the 
battery 173 and the solenoid 146 of valve 142 through a path defined by 
conductor 174, the photo switches 126, conductor 188, contacts 190, 
conductor 202, contacts 200, and conductors 204, 206 and 176. 
When solenoid 146 of valve 142 is energized, it steps to its condition 
shown in FIG. 8 wherein conduit 152 is coupled to conduit 230 which in 
turn is connected to the upper ends of shot pin cylinders 84 and to the 
lower ends of shot pin cylinders 102 and 118. In addition, valve 142 
couples conduit 154 to a second conduit 228 which is also connected to the 
opposite ends of each of these shot pin cylinders. Steering valve 148 also 
connects conduit 154 to sump 64. As a result, shot pin cylinders 84 are 
moved downwardly and shot pin cylinders 102 and 118 are moved upwardly as 
shown in FIG. 7 so that each of the first limit switch contacts 178, 180 
and 182 are open to de-energize solenoids 146 of valves 144 and 145 so 
that conduits 156 and 158 are disconnected from conduits 157 and 160. In 
this manner the two-wheel steering cylinders 56 are disconnected from the 
pump 140. 
It will be recalled that retraction of the shot pin cylinders 84 will 
uncouple the two-wheel steering cylinders 56 from their respective front 
forks. In addition, outward extension of the shot pin cylinders 102 will 
couple the front four-wheel steering cylinders 86 to the front forks and 
retraction of the shot pin cylinders 118 will uncouple the rear forks from 
the columns 13 to permit four-wheel steering of the rear forks. 
When each of the shot pin cylinders 84 and 118 reach their fully retracted 
positions and shot pin cylinders 102 their fully extended positions, as 
shown in FIG. 9, they will respectively engage a second one of their 
associated limit switches 240, 242 and 244, respectively. It should be 
noted that limit switch contacts 178, 180 and 182 are not shown in FIG. 9 
because they are inactive during the four-wheel mode of operation, but 
they are of course, present in the apparatus. Operation of these limit 
switches will close their associated second limit switch contacts 246, 248 
and 250 thereby completing an energizing circuit for battery 173 to each 
of the solenoids 147 of valves 144 and 145 through a circuit defined by 
said contacts and conductors 174, 175, 252 and 176. The operation of the 
solenoids 147 places the valves 144 and 145 in the condition shown in FIG. 
9 wherein conduits 156 and 158 are respectively connected to flow dividers 
254 and 256 by conduits 258 and 260. The first flow divider 254 is 
connected to one end of steering cylinder 86R by conduit 262 and to one 
end of steering cylinder 104L by conduit 264. Similarly, flow divider 256 
is connected to one end of steering cylinder 86L by conduit 266 and to one 
end of steering cylinder 104R by conduit 268. The opposite ends of 
steering cylinders 86R and 104R are interconnected by conduit 270 and the 
opposite ends of steering cylinders 86L and 104L are interconnected by 
conduit 272. The closing of the limit switch contacts 246, 248 and 250 
also completes an energizing circuit to the relay coil 192 to step the 
relay contacts 190 from their position shown in FIG. 7 to their position 
in FIG. 9. This de-energizes relay 186 which returns contacts 200 to their 
position in FIG. 9 whereby solenoid 146 of valve 142 is de-energized 
whereby the valve returns to its neutral position. 
As indicated above, the steering valve 148 may be manipulated by an 
operator when turning the steering wheel 162 to selectively connect valves 
144 and 145 to pump 140 or sump 164. Because equal amounts of hydraulic 
fluid will thus be delivered to each cylinder by the action of flow 
dividers 254 and 256, in the ideal case, each wheel is turned in the same 
direction and through the same angle. However, unavoidable variables such 
as valve and cylinder leakage, flow divider efficiency and slight linkage 
misalignment may cause slight differences in the hydraulic fluid delivered 
to each of the cylinders or slight variations in the turning angle as a 
result of mechanical differences. A servo system 274 has, therefore, been 
provided to equalize the turning angle of all wheels. The system 274 
includes four potentiometers which 138RF, 138LF, 138RR and 138LR are 
mounted respectively in the yokes 42RF, 42LF, 42RR and 42LR. It will be 
recalled with reference to FIG. 6 that each of the potentiometers 138 are 
supported on their respective forks and that each of the wipers 140 are 
mounted on the adjacent column. Accordingly, if a potential was applied 
across each of the potentiometers, the voltage at the wiper will provide a 
measure of the deflection of each yoke. This relationship is employed to 
provide error signals which indicate the deviation, if any, in the 
deflection angle of the right front and two rear yokes with respect to the 
left front yoke by comparing the voltage at each of the potentiometer 
wipers 140RF, 140LR and 140RR with that at 140LF. Toward this end, a 
single conditioning and error amplifying signal 276 is associated with 
each potentiometer for receiving the wiper potential. 
The servo circuit 274 also includes three voltage comparator circuits 278, 
280 and 282. The voltage output signal from the circuit 276LF associated 
with the left front fork is provided as one input to each of the voltage 
comparator circuits 278, 280 and 282. The second input for each of the 
voltage comparator circuits 278, 280 and 282 are respectively the voltage 
output signals from the circuits 278LF, 276LR and 276RR. Accordingly, 
comparator circuit 278 compares the voltage representing the deflection of 
the right front fork 42RF as provided by circuit 276RF and the deflection 
of the left front fork 42LF as provided by the circuit 276LF. Similar 
comparisons are made by the circuits 280 and 282 of the deflections of the 
left rear and right rear forks to the right front fork. Coupled to voltage 
comparator circuit 278 is a first driver circuit 278a which is actuated 
when the signal from the circuit 276RF exceeds the signal from the circuit 
276LF and a second driver circuit 278b which is actuated when the signal 
from circuit 276RF is less than the signal from circuit 276LF. Similarly, 
driver circuits 280a and 280b are connected to voltage comparator circuit 
280 and driver circuits 282a and 282b are connected to voltage comparator 
circuit 282. Each driver circuit identified by the lower case "a" is 
actuated when its associated potentiometer signal exceeds that of the 
master potentiometer 130LF and the driver circuits identified by the lower 
case "b" are actuated when the associated potentiometer signal is less 
than that of the master potentiometer 138LF. 
Associated with cylinders 86R, 104R and 104L are valves 86RV, 104RV and 
104LV, respectively. Each of the valves 86RV, 104RV and 104LV has a first 
solenoid 288 connected, respectively, to the driver circuits 278a, 280a, 
282a and a second solenoid 290 respectively connected to driver circuits 
278b, 280b and 282b. It will, therefore, be appreciated that when the 
voltage at wiper 140RF exceeds the voltage at wiper 140LF indicating that 
the right front fork 42RF is ahead of the left front fork 42LF the driver 
circuit 278a will receive a signal to actuate solenoid 288. Conversely, 
when the voltage on the wiper 140RF is less than the voltage in wiper 
140LF indicating that the right front fork is behind the left front fork, 
driver circuit 278b will receive a signal to actuate solenoid 290. The 
solenoids 288 and 290 of valves 140RV and 140LV will similarly be actuated 
if the left rear or right rear forks are ahead of or behind the left front 
fork. 
When solenoid 288 of valve 86RV is actuated, it will connect conduit 292 to 
conduit 294 thereby providing a return path to the sump from the upper end 
of cylinder 86R. This will tend to cause the right front fork to move back 
into annular alignment with the left front fork. On the other hand, should 
the right front fork fall behind the left front fork, driver circuit 278b 
will be actuated to energize solenoid 290 of valve 86RV. As a result the 
lower end of cylinder 86R will be connected to sump 64 through conduits 
296 and 298. This will tend to increase the rotation of the right front 
fork and to move the same into alignment with the left front fork. As the 
right front fork moves into alignment with the left front fork the signal 
from circuit 278 will cease to de-energize solenoid 288 to return valve 
86RV to its neutral position. When the forks are in fact in alignment, the 
signal to driver circuits 278a will cease thereby de-energizing solenoid 
290 and opening the shunt path through conduits 296 and 298. The valves 
104RV and 104LV will similarly be actuated should the left rear or right 
rear forks move out of alignment with the left front fork. 
When it is desired to switch the hydraulic system from the four-wheel 
steering mode to the two-wheel steering mode, each of the wheels are 
returned to their zero deflection position which will close the photo 
switches 126. Selector switch 184 may then be closed to complete the 
energizing circuit to the solenoid 299 through conductors 174, contact 
299, conductors 300, 302 and 176. This steps relay contacts 304 from their 
position shown by full lines to that shown by broken lines in FIG. 9 to 
energize solenoid 147 of valve 142 through conductors 174, 305, 306, 308 
and 176. When solenoid 147 is energized, it operates valve 142 so that 
pump output conduit 152 is connected to conduit 228 and sump return 
conduit 158 is connected to conduit 230. In this manner, the upper end of 
shot pin cylinders 102 and 118 at the lower end of shot pin cylinders 84 
are connected to pump 140 and their opposite ends are connected to pump 
164. Accordingly, each of the shot pin cylinders 84 are extended to couple 
the two-wheel steering cylinders 56 to the front forks, shot pin cylinders 
102 are retracted to uncouple the four-wheel steering cylinders 86 from 
the front forks and the shot pin cylinders 118 are extended to couple the 
rear forks to the rear posts 42. This movement of the shot pin cylinders 
will open each of the limit switch contacts 246, 248 and 250 and 
de-energize solenoids 147 so that valves 144 and 145 are returned to a 
neutral position to prevent the delivery of fluid under pressure to the 
four-wheel steering cylinders 86 and 104. When each of the shot pin 
cylinders reaches the end of their travel, the limit switch contacts 178, 
180 and 182 are closed as shown in FIG. 7 so that the two-wheel steering 
cylinders 56 may then be initiated in the manner discussed with respect to 
FIG. 7. 
The closing of the contacts 178, 180 and 182 will also energize relay 310 
to step contacts 190 from their position shown in FIG. 8 to their position 
shown in FIG. 7. As a result relay 299 is de-energized to de-energize 
solenoid 147 of valve 142 which then returns to the neutral position shown 
in FIG. 7. 
While only a single embodiment of the invention is illustrated and 
described, it is not intended to be limited thereby but only by the scope 
of the appended claims.