Method and apparatus for supporting a turf mower cutting unit

One aspect of the invention relates to a cutting unit joint (22) which can be located between a lift arm (18) and a cutting unit (20). Cutting unit joint (22) includes a four-bar linkage preferably including pairs of pivoting links (46 and 48) which create a "virtual pull point" at any desired location. With regard to one embodiment, a cutting unit (20), the virtual pull point is even located below ground, when the cutting unit (20) is generally level. This configuration reduces or eliminates cutting unit "bobbing." In another embodiment, in connection with a cutting unit (20'), the virtual pull point is located roughly at the centerline of the reel, to maintain a good "seal" between the cutting unit (20') and a lift arm supported grass basket (52'). Another aspect of the present invention relates to a self-contained damper (76) connected to lift arm (18), the damper (76) resisting upward movement at a velocity or frequency consistent with cutting unit "bobbing."

TECHNICAL FIELD 
The invention relates generally to turf maintenance equipment, and more 
particularly to a method and apparatus for mounting a cutting unit to the 
traction vehicle of a power turf mower. 
BACKGROUND OF THE INVENTION 
Many types of power turf mowing equipment are known. Such equipment can 
generally be classified as those mowers which include a rotary cutting 
unit(s) or those which include a reel cutting unit(s). A rotary cutting 
unit usually includes one or more rigid, generally straight, steel blades 
rotated within a housing about a vertical axis to sever grass or other 
vegetation at a predetermined height above the ground. A reel cutting 
unit, on the other hand, typically includes a frame within which is 
horizontally rotatably mounted a reel possessing a plurality of arcuate 
blades. The rotating reel blades pass in close proximity to a bedknife 
which spans the cutting unit frame parallel to the horizontal reel axis. 
Grass blades are sheared at the reel blade/bedknife interface. 
While rotary cutting units are suitable for many purposes, it is generally 
perceived that reel units provide a higher quality cut. The present 
invention relates primarily to turf reel mowers, and the remainder of this 
discussion will focus on same. As is well known, another reel-like unit is 
used to produce a "verticut," and this type of cutting unit is also 
contemplated. 
Power turf mowing equipment can also be categorized based on the method of 
propelling the cutting units. Generally, there are walk-behind, riding, 
and towed turf mowers. The present invention relates primarily though not 
exclusively to riding turf mowers, and for the sake of brevity the 
invention will hereafter be discussed in terms of commercial riding mowers 
having one or more reel cutting units. 
Riding reel-type turf mowers typically include a traction vehicle supported 
by a plurality of wheels, including one or more traction wheels; a prime 
mover supported by the vehicle frame and connected through a transmission 
to the traction wheel(s); one or more reel-type cutting units, powered by 
the prime mover through belts or hydraulic motors, for example; one or 
more cutting unit lift arms pivotally mounted to the traction vehicle; a 
"pivot joint" between each lift arm and the vehicle; and a "cutting unit 
joint" between each cutting unit and its corresponding lift arm. Over the 
years, much effort has been directed to improving the quality of cut 
afforded by reel-type mowers. These efforts have been concentrated 
primarily in two areas, namely the cutting units themselves and the 
technique by which the cutting units are connected to the traction unit. 
The present invention relates in particular to a unique cutting unit 
suspension technique which can address any of a wide variety of cutting 
unit support problems, including but not limited to cutting unit 
"bobbing"; and providing a good "seal" between a floating cutting unit and 
a lift arm supported grass catcher. Each of these particular examples of 
problems which can be addressed by the cutting unit suspension technique 
of the present invention is discussed below. 
Cutting Unit Bobbing 
One absolutely critical capability of reel-type riding mowers, particularly 
those for use on golf courses, is to be able to cut the turf with great 
precision. That is, the height of cut must be extremely consistent across 
the entire width of the mower, regardless of the terrain. This requires 
that the cutting unit(s) "float" relative to the traction vehicle. A truly 
free-floating reel unit can follow the contours or undulations of the 
ground irrespective of the relatively gross motion of the traction 
vehicle. If reel units are not supported in a floating manner, they tend 
to scalp the higher regions and skip over the lower regions. 
The pivot joint typically allows for one "degree of freedom" of the cutting 
unit: if the cutting unit needs to move up or down, relative to the 
traction vehicle, to follow turf undulations, the lift arm pivots on the 
traction vehicle to permit this to happen. But it is the cutting unit 
joint, the joint between the lift arm and the cutting unit, that is 
usually called upon to provide most of the flexibility needed to 
accommodate a more or less free-floating cutting unit. That is, the 
cutting unit joint typically provides for the other degrees of freedom 
often associated with a high precision floating cutting unit of the type 
used on golf courses and similarly demanding locations. These other 
degrees of freedom can include "rolling" (pivoting side to side about a 
longitudinal axis); "yawing" (pivoting about a vertical axis); and 
"pitching" (pivoting about a lateral axis), to make use of aviation terms 
to describe various motions of a reel-type cutting unit as it follows the 
turf. 
While floating the reel unit(s) is desirable, for the reasons set forth 
above, it can be carried too far. If the connection between the reel unit 
and the lift arm is too "floppy," it can cause the reel unit to be 
difficult to handle in its raised transport position; and, it is believed, 
can also contribute to "bobbing" of the reel unit in some conditions. 
Bobbing takes place when the rearward resistance imposed by the turf on 
the front roller of the cutting unit causes the cutting unit to "pitch" 
downwardly due to the manner in which the cutting unit is suspended from 
the lift arm. Once the turf is sufficiently compressed by the front 
roller, in a spring-like manner, the front roller of the cutting unit is 
vaulted upwardly a short distance, and the pattern repeats itself over and 
over in an oscillating fashion. This up and down oscillation, or 
"bobbing," can create subtle but unacceptable ripples in the cut swath. 
The degree to which bobbing occurs is thought to depend on a variety of 
factors, including the mower speed, the manner in which the reel is 
connected to and supported by the traction vehicle, and the turf 
conditions. 
In order to provide cutting unit floating while at the same time preventing 
cutting unit bobbing, attempts have been made to try to place more of the 
weight of the cutting unit on the rear roller to reduce the pitching 
forward effect described above. For example, The Toro Company, assignee 
herein, has used springs acting between the lift arm and the cutting unit 
to cause greater pressure on the rear roller and less pressure on the 
front roller of the cutting unit. Similarly, John Deere has attached the 
lift arm toward the rear of the cutting unit, again to reduce the pressure 
on the front roller. Both of these techniques are somewhat helpful in that 
the front roller will not tend to pitch forward as much, depending of 
course on the turf conditions and the forward speed of the traction 
vehicle. But in both cases, bobbing can still occur since at some point 
the cutting unit will nose (pitch) downwardly once the rolling resistance 
gets to a point where the cutting unit tends to rock or tip forwardly. 
Attempts have also been made to address the bobbing problem at the pivot 
joint. The Toro Company, assignee herein, has tried using spring down 
pressure on the lift arm to improve ground following (see U.S. Pat. No. 
5,042,236), and at least one company has seemingly attempted to compensate 
for bobbing oscillations using the lift cylinder(s) and associated 
hydraulic system (i.e., pump(s), valves, etc.). While these attempts 
directed not to the cutting unit joint but rather to the pivot joint might 
partially control bobbing, they do not control it appreciably. 
An important aspect of the present invention is a cutting unit support 
technique that addresses the bobbing problem in a systematic way, which in 
a preferred embodiment involves the pivot joint as well as the cutting 
unit joint. 
"Seal" Between Grass Catcher and Cutting Unit 
As discussed above, an important function of a typical cutting unit joint 
is to help "float" the cutting unit in a controlled manner. But there are 
other problems that can or must be addressed by the cutting unit joint(s). 
For example, the cutting unit joint(s) must, for some mowers, accommodate 
a grass catcher. In many high precision reel mowers used on golf courses, 
for example, the grass catcher is actually supported by the lift arm, not 
by the cutting unit. The theory is that the cutting unit would cut 
inconsistently if it had to bear the variable weight of the grass catcher 
(i.e., variable by virtue of the fact that the catcher becomes heavier as 
it becomes filled with grass clippings). In such high precision mowers, 
the manner in which the lift arm supports the cutting unit and the grass 
catcher must permit sufficient floating of the cutting unit, to allow it 
to accurately follow the ground undulations, while at the same time 
ensuring a good "seal" between the cutting unit and the grass catcher. 
Otherwise, clippings would not be efficiently collected, and an 
unacceptably large percentage of same would end up on the turf rather than 
in the grass basket. 
The present invention includes a cutting unit joint which can provide a 
good "seal" between the cutting unit and the grass catcher, even when the 
cutting unit is "floating" relative to the lift arm to follow ground 
undulations. 
Thus there is a wide variety of problems associated with suspending a high 
precision cutting unit from a traction vehicle. Floating the reel unit, in 
a manner that reduces or eliminates cutting unit bobbing, is one 
particularly important problem. Another is accommodating a grass catcher. 
The cutting unit suspension technique of the present invention addresses 
these two problems, and more. 
SUMMARY OF THE INVENTION 
Accordingly, in broad terms one aspect of the invention is directed to a 
turf maintenance machine including a traction vehicle; a cutting unit; and 
means including a four-bar linkage for connecting the cutting unit to the 
traction vehicle. 
Another aspect of the invention is directed to a turf maintenance machine 
including a traction vehicle; a cutting unit; a cutting unit support 
system connecting the cutting unit to the traction vehicle; and a 
self-contained damper operatively connected to the cutting unit support 
system which prevents sudden movement of the cutting unit consistent with 
cutting unit bobbing. 
In a preferred embodiment, the turf maintenance machine referred to above 
includes a lift arm; a pivot joint connecting the lift arm to the traction 
vehicle; and a cutting unit joint connecting the cutting unit to the lift 
arm, wherein the four-bar linkage is a component of the cutting unit 
joint. 
In another preferred embodiment, the turf maintenance machine(s) referred 
to above having the four-bar linkage also include(s) a self-contained 
damper which prevents sudden upward movement of the lift arm at a velocity 
or frequency consistent with cutting unit bobbing. 
In one embodiment of the present invention, the "virtual pull point" 
established by the four-bar linkage is generally below grade, at least 
when the cutting unit is on generally level ground, whereby cutting unit 
bobbing is substantially eliminated. 
In another embodiment of the present invention, a grass basket is supported 
by the lift arm of the turf maintenance machine, and the virtual pull 
point established by the four-bar linkage is located proximate the 
transverse centerline of the reel, whereby the seal between the grass 
basket and the cutting unit is maintained even as the cutting unit follows 
the contour of the turf.

DETAILED DESCRIPTION OF THE INVENTION 
Referring to the Drawings, wherein like reference numerals designate like 
parts and assemblies throughout the several views, FIG. 1 shows a 
perspective view of a preferred turf maintenance machine 10 according to 
the invention. Machine 10 is preferably a reel-type turf mower generally 
of the type represented by the Reelmaster.RTM. 5300 mower sold by The Toro 
Company, assignee herein; but those skilled in the art will appreciate 
that the present invention could be applied to other types of turf 
maintenance equipment. Mower 10 includes a traction vehicle 12 supported 
by a pair of front drive wheels 14 appropriately coupled through a 
transmission (not shown) to a prime mover (also not shown). A pair of rear 
steerable wheels 16 support the rearward end of vehicle 12. 
Turf maintenance machine 10 can be used in a variety of turf maintenance 
capacities, e.g., mowing and dethatching. Hydraulically-powered tools are 
operatively connected to vehicle 12 for these purposes. A plurality of 
front tool-supporting lift arms 18a are pivotally connected to the front 
central portion of a frame 19 of vehicle 12. Similarly, a plurality of 
rear tool-supporting lift arms 18b are pivotally connected to the frame 19 
of vehicle 12 proximate to its longitudinal mid-point. The front lift arms 
18a are, in a preferred embodiment, actually pivotally coupled to a 
sub-frame sometimes called the tractor carrier frame. The rear lift arms 
18b, by contrast, attach directly to the frame of traction vehicle 12, 
with similar pivot joints. The tractor carrier frame mainly facilitates 
manufactureability, and certainly all of the lift arms 18 could be 
attached directly to the main frame of the traction vehicle 12. 
A front reel unit 20a is coupled to each front lift arm 18a through a 
unique cutting unit joint 22a which includes a "four-bar linkage," as 
further described below. A rear reel unit 20b is coupled to each rear lift 
arm 18b, through the use of a cutting unit joint 22b which is preferably 
substantially identical to cutting unit joint 22a. One aspect of the 
present invention is particularly directed to the cutting unit joints 22, 
and they will be described in some detail below. 
In the preferred embodiment, there are three front cutting units 20a spaced 
along a line generally perpendicular to a fore-and-aft or longitudinal 
axis 23 of mower 10 and directly in front of the drive wheels 14; and two 
rear cutting units 20b roughly centered between vehicle wheels 14 and 16 
and also arranged along a line which is generally perpendicular to the 
longitudinal axis 23 of the mower. As known by those skilled in the art, 
the rear cutting units 20b overlap the front cutting units 20a to handle 
the uncut strips of grass between the front cutting units. 
In the preferred embodiment, the front and rear cutting units 20 and 
related support structures are substantially identical in all pertinent 
respects, and therefore the "a" and "b" suffixes will not hereafter be 
used in most instances. In fact, in a preferred embodiment the front and 
rear cutting units are completely interchangeable. 
Each cutting unit 20 can be raised from a normal operating position to a 
transport position wherein the cutting unit 20 is up out of engagement 
with the turf. Paired with each cutting unit 20 is a hydraulic lift 
cylinder 24, selectively operable by the operator. Each lift cylinder 24 
is arranged such that its cylinder is pivotally secured to vehicle frame 
(or sub-frame) 19, and its piston or rod is similarly secured to the 
corresponding lift arm 18. Each hydraulic lift cylinder 24 is capable of 
lifting its associated reel unit 20 from the operating position to the 
transport position. Reel units 20 are for the most part shown in their 
operating positions in the Drawings, but those skilled in the art will 
recognize that the reel units can be selectively raised for transportation 
purposes and to permit the operator to mow a narrower swath or to 
relatively rapidly move to a different work area, for example. 
Each lift arm 18 is preferably constructed from 2.25 inch O.D., 0.188 inch 
wall thickness, steel tube, and can include a lateral portion 26 suitably 
connected to frame 19 and a longitudinal portion 28 generally 
perpendicular thereto. The lateral portion 26 of each lift arm 18 can be 
contained within a vertical plane which is generally perpendicular to a 
vertical plane containing mower longitudinal axis 23. The longitudinal 
portion 28 of each lift arm 18 is generally parallel to the longitudinal 
axis 23 of mower 10. The free end of each lift arm longitudinal portion 28 
carries the associated cutting unit joint 22 which in turn carries the 
corresponding cutting unit 20. The other end of the lift arm, actually one 
end of the lift arm lateral portion 26, is connected to the traction 
vehicle 12 by means of a pivot joint 27. The present invention is 
particularly directed to the pivot joint 27 and the cutting unit joint 22, 
and preferred such joints will be further described below in some detail. 
An enlarged perspective view of the left (as viewed by the operator) front 
reel unit 20 and the preferred cutting unit joint 22 are shown in FIG. 2. 
Although the particular construction of reel unit 20 is not central to the 
invention, the basic components of reel unit 20 will be described. A pair 
of generally rectangular side plates 30 are located at opposite ends of 
the reel unit 20. Plates 30 are spanned at the top by a steel channel 
piece 32 and at the bottom by a rear roller 37 (shown in FIG. 3) and a 
front roller 35. As shown in FIG. 2, the front roller 35 can be a 
so-called "Wiehle" roller, having a series of circumferential grooves 
across its entire width. Also spanning the end plates 30 is a horizontally 
centrally mounted rotatable reel 36 which is coupled to a hydraulic motor 
(not shown in FIG. 2). On the side opposite from the hydraulic motor is a 
counterweight 38, the purpose of which is to provide a counterbalancing 
force as against the hydraulic motor (not shown in FIG. 2; a hydraulic 
motor 39 is shown on the right side of the right front cutting unit in 
FIG. 1), to maintain a consistent turf pressure across the entire width of 
the cutting unit and to minimize "yaw" and "roll" of the cutting unit 
during transport to keep the cutting unit in a substantially level 
orientation. Thus, reel unit 20 is a substantially self-supporting unit 
which is pulled across the ground on rollers, the cutting blade of which 
is powered through a hydraulic connection with the traction vehicle 12. A 
bedknife 59 (shown in the middle drawing of FIG. 3) also spans between end 
plates 30 and is situated proximate the lower outer periphery of reel 36 
so that when reel 36 spins its blades come into close proximity to the 
bedknife to shear the grass blades. 
It should be noted that the "stance" or wheelbase of cutting unit 20 is 
preferably larger than that of the Reelmaster.RTM. 5100/5300, the 
assignee's predecessor machine. The wheelbase of cutting unit 20, i.e., 
the distance between the centerlines of rollers 35 and 37, is 12.25 inches 
or 14.11 inches, depending on the position of the front roller (the front 
roller being carried by a bracket which can be "flipped" from one position 
to another to change the wheelbase). By contrast, the wheelbase of the 
Reelmaster.RTM. 5100/5300 cutting units is 9.66 inches or 11.70 inches, 
again depending on the selected position of a two-position bracket. 
Referring to FIGS. 2, 4 and 5, cutting unit 20 is suspended from a cutting 
unit support frame 40 consisting of a rearwardly opening U-shaped rear 
frame element 42 and a pair of laterally extending, generally straight, 
forward frame elements 44. Frame elements 42 and 44 generally lie in a 
plane which is just below the longitudinal portions 28 of lift arms 18. 
Forward frame elements 44 are located on either side of a steering 
assembly 45 connected to the front central portion of rear frame element 
42. Each forward frame element 44 includes a fixed end welded to the 
forwardmost leg of the "U" of rear frame element 42; and a free end 
located generally above the corresponding side plate 30. Referring to FIG. 
5 in particular, rear frame element 42 has a forwardmost leg 42a in effect 
extending laterally through steering assembly 45, and a pair of rearwardly 
extending legs 42b generally perpendicular to leg 42a which extend 
longitudinally and which terminate toward the rear of the cutting unit 20, 
having laterally outward extensions 42c at the rear to position the 
rearward ends of frame element 42 generally over the rear portion of the 
corresponding side frame 30. Extending downwardly and pivotally from the 
side plate support frame 40 is a pair of forward links 46 and a pair of 
rearward links 48. Links 46 and 48 are pivotally attached to the side 
plates 30 and side plate support frame 40. Cutting unit carrier frame 40 
and pivotal links 46 and 48 collectively comprise a four-bar linkage which 
produces a "virtual pull point" in a location depending on the "focal 
point" of links 46 and 48, as further described below. 
Referring to FIGS. 2 and 3, links 46 act generally in parallel with one 
another, and similarly links 48 act generally in parallel with one 
another. Links 46 and 48 are preferably constructed from cast steel and 
are about 5.75 inches long. The pivot or revolute joints, both at the 
cutting unit frame 40 and at the side plates 30, are constructed in a 
standard manner using bushings, studs and the like. Referring to FIG. 3, 
the actual pivot point 49 (as opposed to the "virtual pull point," 
discussed further below) of each forward link 46 on the corresponding side 
plate 30 is located roughly midway between the centerline 51 of the 
cutting reel and the centerline 53 of the forward roller, fore-and-aft; 
and about midway between the upper edge of each side plate 30 and the 
centerline 51 of the cutting reel, top to bottom. As also shown in FIG. 3, 
the actual pivot point 55 for each rearward link 48 is located about 
midway between the centerline 51 of the cutting reel and the centerline 57 
of the rear roller 37, fore-and-aft; and at about the same vertical 
location as the centerline 51 of the reel. Clearly the actual pivot points 
for the links 46 and 48 could be located in other locations in different 
embodiments of the present invention, since the location of the "virtual 
pull point" (discussed further below) is the key, not the locations of the 
actual link pivot points. It should be noted that links 46 and 48 can be 
simple straight elements as shown in the Drawing, or can alternatively 
have other shapes. Regardless of the shape of a given link, the effective 
link would lie along a straight line extending through the actual pivot 
points. 
The specifics of certain aspects of the preferred cutting unit suspension 
system, not related to the four-bar linkage, are described in co-pending 
patent application entitled "OPERATING UNIT JOINT FOR TURF MAINTENANCE 
EQUIPMENT," filed on even date herewith, assigned to the assignee herein, 
and incorporated herein by reference. 
FIG. 3 shows a somewhat diagrammatic side elevational view of cutting unit 
20 as it traverses a small hill. The middle drawing, in solid line, shows 
cutting unit 20 more or less on level ground. Notably, the "focal point" 
50 of links 46 and 48 is actually located below grade. This focal point is 
the so-called "instant center" or "instantaneous center" of a four-bar 
linkage consisting of cutting unit carrier frame 40 as a first "bar"; 
cutting unit 20 as a second "bar"; and links 46 and 48 as the third and 
fourth "bars." Instant center 50 constitutes the "virtual pull point" for 
cutting unit 20, in the sense that cutting unit 20 will behave, at this 
particular instant of time (i.e., when the four-bar linkage is in the 
particular position shown in the drawing), as if the cutting unit were not 
being pulled by steering joint 45 or links 46 or 48 alone, but rather 
pulled by or at the virtual pull point 50, the location of which 
corresponds with the "focal point" of links 46 and 48. The concept of an 
instant center or instantaneous center of a four-bar linkage is well known 
to engineers; however, as far as Applicants know, this concept has never 
been applied to a turf mower, in order to in effect place the pull point 
for the cutting unit at any desired location. 
As can be seen in FIG. 3, the instant center 50 of the four-bar linkage 
shifts as the cutting unit 20 traverses the hill. It shifts because the 
relative positions of the various links or bars shaft, and this changes 
the focal point of links 46 and 48. On generally level ground, as shown in 
the middle drawing of FIG. 3, the virtual pull point or instant center 50 
is actually located below grade (i.e., underground), generally aligned 
with the centerline 51 of the cutting reel fore-and-aft, or more 
specifically just forward of the leading edge of bedknife 59 of cutting 
unit 20. 
Still referring to FIG. 3, when the cutting unit 20 is on generally level 
ground, the angle 61 between links 46 and 48 is about 62 degrees. When the 
cutting unit is climbing a grade of about 15 degrees measured from the 
horizontal, as shown on the left hand side of FIG. 3 in phantom line, the 
angle 61 between links 46 and 48 is about 69 degrees. When the cutting 
unit 20 is descending a hill of about 15 degrees, measured from the 
horizontal, the angle between links 46 and 48 is about 62 degrees. If 
cutting unit 20 were to traverse a hill as shown in FIG. 3, instant center 
50 would, in a sense, trace an upwardly turned curve or "smile," having 
its low point located as shown in the middle (level ground) drawing of 
FIG. 3, and having the upper corners of the "smile" located as shown in 
the left and right hand drawings of FIG. 3. The shape of the "smile" would 
depend, among other things, on the overall configuration of links 46 and 
48, in terms of their relative lengths, and their actual pivot locations. 
Applicants believe that the shifting of the virtual pull point actually 
enhances ground following. Referring to FIG. 3, when ascending a hill the 
virtual pull point 50 automatically shifts to the rear, proximate the rear 
roller 37. This in effect unloads the front roller 35 so that it isn't 
driven into the side of the hill in such a way as to scalp the high areas, 
but rather is "light" enough so that it can remain responsive to further 
changes in the topography. When descending, the virtual pull point 50 
automatically shifts into proximity with front roller 35, forcing the 
front roller 35 down into the hill so that the cutting unit doesn't tend 
to skip over the low areas. 
One objective, particularly with regard to cutting unit 20, was to 
eliminate "bobbing" by means of the four-bar linkage. That being the case, 
it was decided to locate the "virtual pull point" roughly in line with the 
front edge of the bedknife 59 and slightly below grade, when the cutting 
unit 20 is on generally level ground (as shown in the middle, solid line 
drawing of FIG. 3). While the instant center of the linkage depicted in 
FIG. 3 certainly shifts fore-and-aft and up-and-down, it can be seen that 
the instant center is generally located at or below an imaginary line 63 
passing through the centerlines 53 and 57 of the support rollers of 
cutting unit 20. The rationale behind lowering the pull point, by means of 
a four-bar linkage, is as follows: as noted above, bobbing is caused by 
the cutting unit "pitching" or nosing downwardly due to the rolling 
resistance on the front roller. When this happens, the turf in effect acts 
like a spring, and eventually pushes upwardly on the front roller enough 
to vault the front roller upwardly, and the whole process repeats itself 
over and over again. By locating the "virtual pull point" at a very low 
point, when the cutting unit rollers encounter resistance in the turf, 
this actually causes the front roller not to further compress the turf, 
but rather to pivot in the opposite direction, thus unloading the 
resistance forces from the front roller. Similar unloading occurs when 
resistance forces are encountered at the rear roller. The net effect is a 
system that seeks equilibrium. Thus, cutting unit 20 in effect behaves as 
if it were pivoting about a point located quite low, even below grade in 
some circumstances; and "bobbing" is considerably reduced if not 
eliminated. 
Links 46 and 48 still permit controlled "pitching" in order to accommodate 
ground following as much as possible. Although the "pitching" movement 
created by the four-bar linkage is more complex than a simple pivoting 
motion produced by prior art cutting unit suspensions, there is still 
sufficient ground following, in the "pitching" mode, to prevent scalping 
or unmowed low areas. This complex movement is defined by the linkage 
geometry, and requires more vertical motion of the lift arm to occur than 
is required by single pivot systems, to traverse an equivalent ground 
contour. That is, cutting unit 20 is not permitted to undergo a simple or 
pure "pitching" movement in response to undulations in the turf. Rather, 
cutting unit 20 "rocks" fore-and-aft at the same time it pitches upwardly 
and downwardly, as depicted in FIG. 3, and this restraint on the motion of 
cutting unit 20 translates into an upward or downward force on 
corresponding lift arm 18. This vertical force, if upward, is vigorously 
opposed by a damper 76 (shown in FIGS. 12 and 13) if the velocity or 
frequency of the upward movement is within the range of velocities or 
frequencies against which damper 76 acts, as further described below. 
Thus, in the most preferred embodiment the four-bar linkage addresses the 
bobbing problem in conjunction with the damper 76. While either the 
four-bar linkage or damper 76 could act alone to help reduce cutting unit 
bobbing, in the most preferred embodiment they work together. As an 
example of how these components or systems could work alone, a damper 
could be used in conjunction with a simple cutting unit joint of the type 
which provides, for example, only "rolling" of the cutting unit. In that 
case, bobbing inducing forces would obviously tend to cause vertical 
movement of the lift arm, and the damper would come into play. As another 
example, a four-bar linkage operating alone, without a damper, can reduce 
bobbing even in the absence of a damper at the pivot joint, if the 
"virtual pull point" is located low enough to cause the cutting unit to 
seek equilibrium in such a way that the resistance forces on the rollers 
are automatically unloaded. 
FIG. 6 again shows cutting unit 20 on substantially level ground, and 
illustrates the below grade virtual pull point 50. This can be contrasted 
with an alternative embodiment 20', shown in FIG. 7, wherein the virtual 
pull point 50' is located considerably higher. FIG. 7 illustrates, 
diagrammatically, an entirely different cutting unit 20', wherein a major 
concern is the "seal" between the cutting unit 20' and a grass basket 52' 
supported by the corresponding lift arm. The "seal" between cutting unit 
20' and the grass basket 52' is a concern because the cutting unit 20' 
must be permitted to move relative to the lift arm, whereas the grass 
basket 52' is fixed to the lift arm. This is because, as described above, 
the grass basket can be quite heavy and is of variable weight. If it were 
supported by cutting unit 20', this would cause cutting unit 20' to have a 
variable weight, depending on the amount of grass clippings in grass 
basket 52'. This would cause the quality of cut and height of cut of 
cutting unit 20' to vary depending on the amount of grass clippings in 
grass basket 52', and also cause excessive compaction of the turf in some 
situations. FIGS. 8 and 9 show cutting unit 20' ascending and descending a 
small hill, respectively. As can be seen, the cutting unit 20' pivots 
considerably relative to grass basket 52'. However, cutting unit 20' would 
move even more were a standard cutting unit suspension system used. In 
most cases, with prior art cutting unit suspensions, the cutting unit 
would move away from the grass basket such that the "seal" would be lost 
and clippings would tend to fall to the ground. The rationale behind using 
the four-bar linkage system in connection with cutting unit 20' is to 
attempt to locate the virtual pull point 50' as close as possible to the 
centerline of the cutting reel, designated 51' in FIGS. 8 and 9. If the 
virtual pull point 50' were located directly on the centerline of the 
cutting reel, then the "seal" between the cutting unit 20' and the grass 
basket would be optimized. However, because of physical constraints, it is 
not always possible to arrange links 46' and 48' in such a way that they 
would "focus" directly on the reel centerline 51'. As shown in FIGS. 1 and 
2, the side plates 30 of typical reel-type cutting units are quite crowded 
in terms of components and fasteners. This being the case, it is sometimes 
impossible to locate the actual pull point of a cutting unit in any 
particular location. That highlights one significant advantage of the 
four-bar linkage system of the present invention: by using pivoting, 
"focused" links to create a virtual pull point, the suspension elements 
and links for the cutting unit can be located where convenient, in effect, 
whereas the virtual pull point can be established by focusing the links 
anywhere on or off the cutting unit, even below grade (as discussed above 
in connection with the first embodiment). Arrows 54 generally represent 
the flow direction of the grass clippings produced by cutting unit 20'. It 
can be seen that in all cases the clippings flow into grass basket 52', 
attributable to the good "seal" between the cutting unit 20' and grass 
basket 52'. 
With reference to FIGS. 7, 8 and 9, links 46' act generally in parallel 
with one another, and similarly links 48' act generally in parallel with 
one another. Links 46' and 48' are preferably constructed from cast steel 
and are about 4.75 inches long. The pivot or revolute joints, both at the 
cutting unit frame 40' and at the side plates 30', are constructed in a 
standard manner using bushings, studs, and the like. In this embodiment, 
the actual pivot point 49' (as opposed to the "virtual pull point") of 
each forward link 46' on the corresponding side plate 30' is located 
closer to the centerline 51' of the cutting reel; but is still located 
about midway between the upper edge of each side plate 30' and the 
centerline 51' of the cutting reel, top to bottom. This is in contrast to 
the suspension for cutting reel 20, shown in FIG. 6. Further with 
reference to cutting unit 20', the actual pivot point 55' for each 
rearward link 48' is located closer to the centerline 51' of the cutting 
reel; and at about the same vertical height or location as the front pivot 
point 49'. When the cutting unit 20' is on generally level ground, as 
shown in FIG. 7, the angle 61' between links 46' and 48' is about 38 
degrees. When the cutting unit 20' is climbing a grade of about 15 degrees 
measured from the horizontal, as shown in FIG. 8, the angle 61' between 
links 46' and 48' is about 42 degrees. On the other hand, when the cutting 
unit 20' is descending a hill of about 15 degrees, measured from the 
horizontal, as shown in FIG. 9, the angle between links 46' and 48' is 
about 39 degrees. 
It should be noted that the link angles given above are with respect to the 
front two cutting units of a so-called "triplex" mower; the rear cutting 
unit is preferably supported in such a way that the angle 61' between 
links 46' and 48' on level ground is about 50.degree., the difference 
simply being attributable to the fact that the links 46' and 48' connected 
to the side plates in a somewhat different manner in the case of the rear 
cutting unit. But even in this case, the virtual pull point is located 
near the centerline of the reel, for the reasons set forth herein. 
As in the case of cutting unit 20, discussed above, if cutting unit 20' 
were to traverse a hill, as collectively shown in FIGS. 7, 8 and 9, 
instant center 50' would, in a sense, trace an upwardly turned curve or 
"smile," having its low point located as shown in FIG. 7, and having the 
upper corners of the "smile" located as shown in FIGS. 8 and 9. Note that 
in the case of cutting unit 20', the "smile" is considerably less 
pronounced as compared to the "smile" created or traced by virtual pull 
point 50 of cutting unit 20. Also, the "smile" of cutting unit 20' is much 
narrower than the "smile" of cutting unit 20. Again, the size and shape of 
the "smile" would depend, among other things, on the overall configuration 
of links 46' and 48', in terms of their absolute and relative lengths, and 
their actual pivot locations on side plates 30' and cutting unit frame 
40'. In the case of cutting unit 20', it was desired to keep the virtual 
pull point 50' as close as possible to the centerline 51' of the cutting 
reel, to maintain a good "seal" between the cutting unit 20' and grass 
basket 52' as the cutting unit 20' pitches relative to grass basket 52'. 
In the case of cutting unit 20, it was desired to create a very low pull 
point on level ground, to minimize bobbing. Cutting unit 20 is used on a 
mower which typically operates at a higher speed than a mower which would 
use cutting units 20', and thus bobbing with regard to the former is more 
of problem. 
FIGS. 10 and 11 point out a still further advantage to the four-bar linkage 
suspension system described in connection with cutting unit 20'. FIG. 10 
shows the cutting unit 20' in its transport position, i.e., raised up off 
of the ground. Cutting unit 20' hangs essentially level since the center 
of gravity of the cutting unit, designated with reference numeral 58' in 
FIG. 10, is located directly above the virtual pull point 50' of the 
cutting unit 20'. As such, there is no moment created by the weight of 
cutting unit 20' which would tend to pitch it forwardly or rearwardly. In 
fact, if there is a moment which would tend to rock the cutting unit 20', 
it will move in such a direction that the center of gravity will in effect 
position itself over the virtual pull point, thus stabilizing the cutting 
unit 20'. This is in sharp contrast to the prior art cutting unit shown in 
FIG. 11, which has pitched dramatically backwards. While FIG. 11 is not 
intended to depict any particular prior art cutting unit, it does 
generally show a typical prior art cutting unit. Note that the actual pull 
point of the cutting unit of FIG. 11 is located at the point labelled with 
reference numeral 60, and the center of gravity of the cutting unit is 
located at 62. This of course causes the cutting unit to rock or pitch 
dramatically backwards, in a counterclockwise direction. Thus, 
manufacturers of cutting units having this type of suspension have to 
include mechanical "stops" which cause mechanical interference between the 
cutting unit and the cutting unit carrier frame, so as to limit the 
dramatic counterclockwise (as viewed in FIG. 11) rotation of the cutting 
unit when it is raised into its transport position. The primary 
disadvantage of the situation depicted in FIG. 11 is that, for a given 
degree of lift provided by the lift arm, the lowest point of the cutting 
unit has not been raised terribly far off of the ground. The space between 
the cutting unit depicted in FIG. 11, labelled 64, is obviously 
considerably less than the space labelled 66 in FIG. 10, whereas in both 
cases the lift arm expended about the same amount of energy in terms of 
lifting. 
FIGS. 12 and 13 show a preferred pivot joint for a lift arm. The pivot 
joint 27 includes the vehicle frame or sub-frame 19, pivotally supporting 
lift arm 18 or, in the most preferred embodiment, lateral portion 26 
thereof. A hydraulic cylinder 24, coupled at one end to frame 19 and 
coupled at the other end to lift arm portion 26, selectively raises the 
cutting unit into its transport position, and selectively lowers the 
cutting unit into its operating position, depending on the wishes of the 
operator. 
Referring to FIGS. 12 and 13, it can be seen that the cylinder of hydraulic 
cylinder 24 is connected to frame 19, and the piston or rod 70 of 
hydraulic cylinder 24 is connected to lateral portion 26 of lift arm 18. 
Actually, there is an element 72 located atop portion 26, forming a slot 
74, and the free end of rod 70 is slideably received by slot 74. This 
creates a "lost motion" connection between hydraulic cylinder rod 70 and 
lift arm 18, such that lift arm 18 can be raised or lowered with 
sufficient contraction or expansion of hydraulic cylinder 24, but wherein 
lift arm 18 can freely float relative to rod 70, by virtue of the rod 70 
freely sliding in slot 74, during normal cutting operation. FIG. 12 shows 
the relative positions of the components of joint 27 when the cutting unit 
is in its operating position, whereas FIG. 13 shows the relative positions 
of the components when the cutting unit is in its transport position. 
In addition to the hydraulic cylinder 24, there is a self-contained damper 
76 interconnecting frame or sub-frame 19 and lift arm 18. Damper 76 is 
self-contained in the sense that it is not part of the overall hydraulic 
system of the mower. That is, damper 76 is preferably not simply part of 
the lift cylinder, for example, but is instead a separate element 
altogether. In a preferred embodiment, damper 76 includes a cylinder 78 
which is pivotally connected to an element 80 rigidly fixed to frame 19; 
and a rod 82 pivotally connected to a tab 84 rigidly fixed to lift arm 18. 
In a preferred embodiment, damper 76 is similar to an automotive shock 
absorber, except that the preferred damper preferably does not include a 
spring element, and is primarily if not exclusively a "velocity dependent" 
component in the sense of providing virtually pure damping. The preferred 
damper 76 also has the feature that it provides significant resistance 
against upward movement of lift arm 18, at certain velocities, but 
essentially allows free fall of lift arm 18 in the downward direction. A 
preferred such damper is made by Arvin Ride Control Products, Inc., 
manufacturer of Gabriel shock absorbers, and is sold as part number 
60531540. It should particularly be noted that a damper could be located 
even as a part of the cutting unit suspension, rather than in the pivot 
joint area. It is contemplated that, for example, a damper could be 
positioned between links 46 and 48 such that relative motion between these 
links could be "absorbed" by the damper, to limit bobbing of the cutting 
unit. Or, a damper could be positioned between one of the links 46 or 48 
and one of the other links of the four-bar linkage. It has been found that 
bobbing occurs at about 6 to 15 cycles per second in a typical turf mower 
at about 4 to 8 miles per hour, creating a "wavelength" of about 9 to 12 
inches. Ground following, on the other hand, occurs at frequencies about 4 
cycles per second, or less. Thus, the damper can be chosen to resist 
frequencies or velocities normally associated with bobbing, and permit, 
relatively speaking, frequencies or velocities consistent with normal 
ground following. 
FIGS. 14 and 15 illustrate an alternative embodiment 40' for the cutting 
unit carrier frame. The primary reason for the differences between the 
second embodiment shown in FIGS. 14 and 15 versus the first embodiment 
shown in FIGS. 4 and 5 is attributable to the fact that the second 
embodiment is thought to be somewhat easier to manufacture. In both cases, 
the purpose of the static portion of the cutting unit carrier frame 40 or 
40' is simply to locate stationary pivot points in convenient places 
relative to links 46 and 48. 
It should especially be noted that it is not necessary to have links 46 and 
48 precisely as shown and described above. In fact, there could be, for 
example, fewer or more than four pivoting support links. For example, 
there could be a single pivoting link located toward the rear of a given 
cutting unit, with a pair of pivoting links located toward the front 
thereof. Those skilled in the art of linkage design will fully appreciate 
that the four-bar linkages shown and described above are exemplary, only. 
Preferred embodiments of the invention are described above. Those skilled 
in the art will recognize that many embodiments are possible within the 
scope of the invention. Variations and modifications of the various parts 
and assemblies can certainly be made and still fall within the scope of 
the invention. Thus, the invention is limited only to the apparatus and 
method recited in the following claims, and equivalents thereto.