Cutter disc assembly for rotary lawn mower

A cutter disc assembly for a rotary lawn mower in which the disc has a circular central portion and a peripheral portion with a shallow vertical wall in between. Radial openings are provided in the wall in register with radial grooves formed in the underside of the peripheral portion. A plastic pin is in register with each opening, with the shank of the pin occupying the associated groove and with a stiffly resilient tip portion projecting beyond the periphery of the disc into cutting engagement with the grass. Each groove has a curved rear wall against which the pin bends as the projecting tip of the pin strikes an obstruction as well as a curved front wall against which the pin bends upon rebound, the curved walls being of outwardly flaring shape thereby to support the shank of the pin substantially free of concentrated stress. Each groove has, in addition, a bridge or "ceiling" which serves as a guide surface to hold the pin in a downwardly-angled working position and to provide frictional damping. Protection of tip portion of the pin in its bent state is offered by a ledge or radial extension on the disc either above or below the pin and which "wipes by" the obstruction, shielding the pin from abrading action. Upwardly swept radial vanes are provided above the periphery of the disc to generate air currents which agitate the grass as it is being cut and to provide a stream of air to move the clippings through the discharge chute.

A safe cutter disc assembly for rotary lawn mower is disclosed in Fisher et 
al. application, Ser. No. 611,347 filed Sept. 8, 1975 and now abandoned 
and replaced by Ser. No. 766,377 which has issued as U.S. Pat. No. 
4,065,913. Such assembly includes a disc formed of resiliently deformable 
plastic providing radially extending openings, with cutter pins registered 
in the openings, each cutter pin being formed of durable plastic material 
having a flexible tip which projects beyond the periphery of the disc into 
cutting engagement with the grass. A curved backstop surface behind each 
of the pins supports the shank of the pin for distributed bending to avoid 
setting up concentrated stress. It is the purpose of the present 
application to cover an improvement upon such construction distinguished 
by increased cutting efficiency and improved pin life. 
Thus it is an object of the invention to provide a cutter disc assembly of 
the type employing a flexible plastic pin as a cutting element in which 
the pin is kept under greater control and more nearly in the desired plane 
of cut notwithstanding centrifugal force and the striking of obstructions. 
It is a general object to provide a cutter disc assembly in which each 
cutter pin is mounted and guided in its movement to produce a greater 
cutting efficiency both in the mowing of lawns and the cutting of weeds 
and light underbrush. It is a related object to provide a cutter disc 
assembly in which each pin is not only guided in a cutting plane but in 
which the lateral motion of the pin is reduced by damping action. 
It is another object of the present invention to provide a cutter disc 
assembly in which a cutter pin is more fully protected against development 
of concentrated stress and against surface abrasion by reason of 
obstructions. More specifically, it is an object of the present invention 
to provide a cutter assembly in which the shank of the pin is nested in a 
downwardly facing groove having curved outwardly flaring rear wall against 
which the pin is bent upon striking an obstruction as well as a curved 
outwardly flaring front wall upon which the pin bends upon rebound, the 
effect in both cases being to distribute the bending to avoid concentrated 
stress which would result in eventual breakage. It is a more specific and 
related object to provide a disc construction in which a pin which has 
been backwardly bent by reason of striking an obstruction is shielded 
against abrasion by the obstruction, with the disc having a projecting 
peripheral ledge which "wipes" harmlessly by the obstruction while the 
bent pin is in a protected position. 
It is another object of the present invention to provide a cutter disc 
which may be readily molded of tough durable plastic, which is of light 
construction employing the plastic material to best advantage, and which 
can be economically manufactured and installed to provide a long useful 
life. 
It is yet another object of the invention to provide a cutter disc assembly 
which includes radially projecting, plastic pins and in which the pins may 
be easily and quickly replaced whenever renewal is necessary. It is a 
related object to provide a cutter disc assembly in which replaceable pins 
are protectively mounted in registering grooves but in which the design of 
the grooves is such as to prevent clogging or packing with grass 
clippings, dust or other debris. 
It is an important and general object of the invention to provide a disc 
assembly for a rotary lawn mower which is effective in cutting of 
vegetation but which has a high degree of safety, both upon direct impact 
with an obstruction and in its relative freedom from throwing of stones or 
other missiles.

While the invention will be described in connection with certain preferred 
embodiments, it will be understood that we do not intend to be limited to 
the particular embodiments shown but intend, on the contrary, to cover the 
various alternative and equivalent constructions included within the 
spirit and scope of the appended claims. 
Turning to FIGS. 1 and 2 there is shown a type of rotary mower in common 
use and to which the present invention is applicable. It includes a frame 
11 in the form of a shallow inverted enclosure having an encircling side 
wall 12, a set of wheels 13, a handle 14 for propulsion and guidance, and 
an engine 15 which may, alternatively, be an electric motor, having a 
vertical drive shaft 16 which is conventionally threaded for receiving a 
clamping screw 17. A discharge chute 18 projects tangentially from one 
side of the housing to dispose of the clippings. 
Secured to the drive shaft in horizontal position is a cutter disc 20 which 
carries the cutting elements. Such a disc, which is preferably molded of a 
durable and resiliently deformable plastic, has a circular central portion 
21 (see also FIGS. 3 and 4), an axially offset, or wall, portion 22, and a 
peripheral portion 23 of relatively thin average cross section extending 
from a drop-off 24 and presenting an outer edge 25. Centered in the disc 
is a mounting hole 26 which may be optionally reinforced by a metal 
reinforcing disc either secured to or molded in the disc structure. 
Evenly spaced openings 27, 28, 29 are provided in the wall portion 22 of 
the disc for receiving outwardly extending cutter pins 30. The cutter pins 
are formed of durable, abrasion resistant plastic, such as nylon. Each pin 
includes a head 31, a shank 32 and a stiffly resilient tip portion 33 
terminating in an outer tip 34. The shank portion 32 is dimensioned to fit 
snugly, cantilever fashion, in a radial opening, for example, the opening 
27. Preferably 25 to 75 percent of the length of the pin extends outboard 
of the disc. 
In accordance with the present invention, the annular outer portion 23 is 
formed, on its underside, with radially extending grooves in register with 
the respective openings for receiving the shanks of the pins, each groove 
having a downwardly angled "ceiling", a narrow neck portion, and a curved, 
outwardly flaring rear wall against which the pin bends as the projecting 
tip strikes an obstruction as well as a curved outwardly flaring front 
wall against which the pin may bend upon rebound, thereby to support the 
shank of the pin with the bending distributed along the length thereof for 
avoidance of concentrated stress. 
Referring to the drawings, the grooves are indicated at 37, 38, 39 in 
respective alinement with the pin receiving openings 27, 28, 29. Taking 
the groove 37 as representative, and referring to FIGS. 3-6, the groove 
has a relatively narrow neck 41 having a width d which just slightly 
exceeds the diameter of the shank of a pin 30. The groove has a curved, 
outwardly flaring rear wall 42 which is of increasing curvature and which 
is tangentially faired into the periphery of the disc. The rear wall 
serves as a support or backstop for the shank of the pin as the tip 
portion of the pin strikes an obstruction, for example, in the form of a 
rock or other weighty piece of debris, R (FIG. 6), with the bending of the 
pin against the curved wall serving to distribute the bend smoothly in the 
shank of the pin so as to avoid the setting up of concentrated stress. The 
groove, in addition, has a front wall 43 adjacent the pin which is curved 
and outwardly flaring so that when the pin 30 is released, tending, 
because of its resilience, to rebound in the forward direction through an 
angle .alpha., the pin bends about the curved front wall 43 with the 
bending, again, being distributed along the length dimension of the pin 
for avoidance of concentrated stress. The flaring curvature and the 
narrowness of the throat 41 produce a groove profile which, viewed 
vertically, is of "trumpet" shape. Since the curvature of the rear wall 42 
causes it to merge smoothly into the periphery 25 of the disc, there is no 
abrupt discontinuity and the outer portion of the pin is supported, in 
bent condition, upon the periphery. 
In accordance with the present invention a substantially horizontal bridge 
is provided closely overlying the pin at the periphery of the disc, for 
the purpose of providing a ceiling surface for holding the pin in 
downwardly angled working position over its range of lateral motion. Thus 
each groove includes an inclined but substantially flat "ceiling" 44 which 
is preferably oriented outwardly and downwardly at a shallow angle thereby 
to provide a superimposed guide surface for not only holding the pin to 
the work but also for damping the lateral movement of the pin. The angling 
of the ceiling from the horizontal, indicated at .beta. in FIG. 4, is 
preferably within the range of 2.degree. to 8.degree.. 
The projection of the tip portion of the pin below disc level tends to 
insure that the cut grass will not drag on the bottom surface of the disc. 
The ceiling of the groove, by its contact with the pin, overcomes the 
tendency of the pin to ride at a higher level, that is, to straighten out 
horizontally as a result of centrifugal force, tending to keep the tip 
portion of the pin at a desired cutting level in spite of variations in 
centrifugal force. The ceiling is preferably of such height h (FIG. 4), 
i.e. the groove 37 has such axial depth, as to contain the shank of the 
pin 30 and thus shield and protect it against abrasion from the underside 
which occurs when the rapidly rotating disc is, during the course of 
mowing, brought against some sharp projection on the surface of the 
ground. 
In accordance with one of the aspects of the present invention the ceiling 
surfaces 44 of the respective grooves 37, 38, 39 are formed by integral 
reinforcing "bridges" 47, 48, 49. Taking the groove 37, and its bridge 47, 
as representative, it will be seen in FIG. 5 that the bridge 47 is a 
region of the outer portion 23 of the disc where the disc has augmented 
thickness t. The region of augmented thickness extends circumferentially 
over a length L which substantially overlaps the side walls of the groove, 
and extends radially from the drop-off 24 to the outer edge 25 of the 
disc. Each bridge thus serves to reinforce the disc to make it uniformly 
strong. 
It is a still further object of the present invention to provide upswept 
radial vanes which are integrally formed on the top surface of the disc 
and which are preferably connected at their outer ends with respective 
ones of the bridges. Thus, referring to FIGS. 3, 4 and 5, vanes 57, 58, 59 
extend from the central portion of the disc radially outwardly at their 
outer ends the trailing edges of the respective bridges. The vanes are of 
special upwardly-swept cross section (FIG. 5) so as to create upward 
movement of the air in the immediate vicinity of the pins. Where the disc 
has no air flow openings the vanes tend to cause a partial vacuum above 
the disc which causes the grass to be agitated as it is being cut to 
increase effectiveness of cut and for efficient discharge. The vanes not 
only create desired air flow as the disc rotates at high speed, but also 
insure that the disc is reinforced in multiple directions, the bridges 
providing circumferential reinforcement adjacent the periphery while the 
connected vanes provide radial reinforcement. 
Preferably the wall portion 22 of the disc is relieved by forming therein a 
circular groove 60 that is defined by two ribs 21', 22" (see FIG. 4) which 
are concentric and radially spaced from one another over most of their 
length with alined openings so that the shank of the pin is gripped at 
axially spaced regions to provide firm cantilevered support with a minimum 
of supporting structure. However, to practice the invention it is not 
necessary for the wall 22 to be continuous or for there to be a single 
central recess for removal and replacement of the pins. If desired, the 
wall portion 22, in which the pins are retained, may be discontinuous and 
the wall may be formed of separate segments 65 formed at one end of an 
individual radial recess 66 provided for each of the pins (see FIG. 8). 
In accordance with a further feature of the present invention there is 
provided, at the point where the rear wall 42 merges with the periphery 25 
of the disc, a radially projecting ledge which extends outwardly beyond 
the tip portion of the pin 30 when it is bent backwardly upon striking of 
an obstruction. Thus, referring particularly to FIGS. 3, 6 and 7 it will 
be noted that the bridge 47, along its trailing edge, is extended radially 
outward to form a ledge 61 which projects beyond the periphery 25 by an 
amount indicated at r (FIG. 3) which approximates the thickness of the pin 
at the point of exit. The ledge is preferably smoothly faired with respect 
to the ledge of the disc at its leading edge 62 to insure that the 
obstruction "wipes by", or is "cammed away" from the pin so that the pin 
is protected from radially applied abrasion. If desired, the ledge 61 may 
be extended upstream and faired, also, at its trailing edge. Since the 
amount of material which forms the periphery of the disc is substantially 
greater than the amount of material used in the tip portion of the pin, 
the disc is well able to absorb the impact and forces of abrasion while 
imparting to the pin a much longer life in the face of rough usage. 
It is one of the features of the present construction that the pins 30 may 
be readily replaced, when worn, without any necessity for demounting the 
disc from the drive shaft. That is to say, all of the pins 30, and 
particularly the heads 31 thereof, are all readily accessible for 
withdrawal into the central recess. To remove a pin it suffices to push 
inwardly on the shank or to pry under the head, until the head has been 
sufficiently unseated to enable the pin to be drawn radially inward, the 
pin bowing slightly in the process. Replacement is a simple matter of 
tapping a new pin in place with a light hammer or even with the side of 
the pliers employed in the removal. 
It will be apparent that the construction which has been described amply 
fulfills the objects earlier set forth. Cutting action is more efficient 
since the pins are all maintained in their downwardly angled cutting 
position, that is, maintained in the preferred "cutting plane" defined by 
the tips of the pins, by reason of the "hold-down" action of the groove 
ceiling 44 which is flat and which thus provides its hold-down function 
over the entire range of swing of a pin from a rearwardly bent position 
assumed while striking an obstruction to a forwardly swung, or rebound, 
position after the obstruction has been safely traversed. Because of the 
curved outwardly flaring rear and front surfaces of the groove which 
engage the respective sides of the pin, bending is distributed over the 
longitudinal dimension of the pin, and there is no point of repeated 
stress concentration at which breakage is likely to occur. Because of the 
radially extending "edges" 61 which act as a shelter for the pins, the 
pins have a substantially longer life. Nevertheless, when a pin must 
finally be replaced, replacement is simple and easy. 
While the shanks of the pins are protectively enclosed in downwardly facing 
grooves, it is found that grooves remain free of crushed grass and other 
debris which might affect the freedom of the pin to bend backwardly and 
forwardly under stress. The reason for such freedom is believed to be the 
fact that each pin, in the normal operation of the mower, constantly works 
back and forth and the grooves, being of outwardly flaring profile, are 
inherently self-shedding. 
In addition to holding the cutter pin 30 (FIG. 4) in a downwardly angled 
position, so that the cutting tends to take place below disc level, the 
ceiling 44 of the groove performs the function of damping the movement of 
the pin. High speed movies taken under actual operating conditions show 
that the pin 30 upon being released from an obstruction does indeed swing 
forwardly, due to its resilience and with a snake-like action, until it 
contacts the front wall 43 of the groove just as illustrated in FIG. 6. 
Contact between the upper surface of the pin and the ceiling 44 of the 
groove, particularly in the region of the periphery, is assured because of 
the leveling action of centrifugal force, that is, the tendency of 
centrifugal force to radially aline the pin with its point of attachment. 
The force of the pin against the ceiling is not great enough to cause 
aggravated wear but is great enough to produce a frictional drag which 
inherently inhibits oscillation of the pin through a wide angle after 
striking an obstruction by reason of the pin's inherent resilience. The 
result is to produce a more even and consistent cutting action in spite of 
striking of obstructions and in spite of the uneven growth of the grass 
being cut. 
It is one of the specific features of the present invention that the wall 
portion 22 which has the annular groove 60 that is defined by concentric 
ribs 22', 22" is intersected by radially spaced recesses in the top 
surface of the disc and which extend downwardly to form radially alined 
saddle openings in the ribs. Thus referring to FIGS. 9 and 10, the disc, 
at each pin location, has recesses 71, 72 which penetrate downwardly into 
the ribs 22', 22" to form alined, pin-receiving saddle openings 73, 74. 
The recesses 71, 72 are shown in profile in FIGS. 11, 13. To reinforce the 
pin-receiving openings 73, 74, the annular groove 60 is radially "bridged" 
at each pin position by reinforcing walls 75, 76. In order to achieve 
clearance of the pin between the reinforcing walls, the underside of the 
groove is provided with an upwardly extending recess 81, shown in section 
in FIG. 12, and which axially overlaps the recesses 71, 72 to define a 
through-opening for the pin having the profile illustrated at 82 in FIG. 
14. 
While the bottoms of the recesses 71, 72 and 81 may be circularly profiled 
for reception of a pin of circular cross section, it is preferred to 
employ a pin of non-circular cross section as indicated at 90 in FIG. 16 
and 17. As here shown the pin is of fluted construction having axially 
extending ribs 91-94, and provided with a head 95 and terminating in a tip 
96. The pin construction is more completely discussed in our co-pending 
application, Ser. No. 756,329 filed Jan. 3, 1977. Such a ribbed pin has a 
number of advantages, but it will suffice for present purposes to say that 
the pin shown in FIG. 16, registered in the ribbed opening 82 shown in 
FIG. 14, is nonrotatable in the opening so that the pin maintains a 
constant and predetermined orientation to provide a relatively narrow line 
of impact with the grass thereby to improve cutting efficiency, while 
being reversible through 180.degree.. By use of alined saddle openings the 
pin may be positively anchored, and tightly cantilevered, using a minimum 
amount of material in the disc. Moreover, because of the saddle openings, 
the pin is gripped over only a limited amount of area, and on alternating 
opposite sides, so that replacement of the pin is much easier than where 
the pin is wedged tight in an opening which encircles the pin. Finally, 
access provided through the lower recess 81 permits entry of a prying tool 
for pin removal in those instances where the head of the pin has become 
damaged. 
The recesses 71, 72 and 81 can be readily formed in a two piece mold having 
mold sections M1, M2 (FIG. 18). The mold section M1 is provided with 
projections 71a, 72a while the lower section has a projection 81a, 
corresponding to the similarly numbered recesses. It is preferred that the 
tips of the projections be of scalloped profile to produce a cruciform 
opening (82 in FIG. 14) for keyed reception of the cross section of the 
pin, but the tips of the projections may be circularly profiled if 
desired. 
In describing the preferred form of the invention (which utilizes a notch 
having an outwardly flaring back wall 42 and an outwardly flaring front 
wall 43 to form a notch of trumpet profile with a ceiling 44 in between) 
no mention has been made of the total area of the ceiling. In accordance 
with one of the aspects of the present invention to ceiling area is 
limited so that it is no more than about two to three times the area of 
the pin projected upon the ceiling. Thus, referring to FIG. 20, where the 
total cross hatched area of the ceiling is indicated at 44, such area 
should preferably not exceed the doubly cross hatched area of the pin 30 
by a factor of more than 2 to 3. The result is to produce a notch which is 
sufficiently wide for adequate flexing of the pin as it strikes an 
obstruction, and upon rebound, yet sufficiently small so that the free 
area is not capable of serving as a pocket or pouch for catapulting small 
stones and other debris. 
In the preferred form of the invention the pin is recessed, as described, 
in a groove formed in the underside of the peripheral portion of the disc. 
However, in accordance with the broader aspects of the invention it is not 
essential to provide a groove, and the invention may be employed in 
modified form as illustrated in FIGS. 21, 22 and 22a where corresponding 
numerals have been employed with addition of subscript "a". Thus the disc 
has a disc-shaped body including a central portion 21a and a peripheral 
portion 23a, the central portion having a rotary drive connection 17a. 
Means indicated at 22a are provided for anchoring the pin 30a in a 
radially extending opening 27a which is angled downwardly at a shallow 
angle, with the tip portion of the pin projecting radially beyond and 
below the periphery of the disc. A backstop surface 42a (FIG. 22) is 
arranged in the path of bending movement of the shank portion of the pin 
to support the pin as the projecting tip portion strikes an obstruction, 
and a corresponding frontstop surface 43a, preferably curved, is placed in 
the path of forward movement of the pin to support it upon rebound. A 
bridge, indicated at 47a, closely overlies the pin at the periphery of the 
disc. The bridge serves to hold the pin in a downwardly-angled working 
position notwithstanding the elevating effect of the centrifugal force, 
while producing a dampening effect upon the pin by reason of frictional 
engagement with the ceiling surface 44a as the pin bends backwardly and 
forwardly below the bridge. 
It is further within the scope of the present invention to provide the 
groove 41 of the preferred embodiment, and in which the pin is recessed, 
with a "floor" thereby enclosing the groove on the underside. Such 
structure is illustrated in FIGS. 23 and 24, in which the "floor" surface 
is indicated at 100 opposed to the ceiling surface 44b, and with all other 
corresponding elements being designated by corresponding reference 
numerals with addition of subscript "b". By enclosing the groove on the 
underside, the groove is converted into a radially extending recess, 
completely enclosing the pin, while permitting swinging movement thereof 
between a backstop surface 42b and a front wall 43b. The floor not only 
prevents entry of small stones which conceivably could become missiles, 
but it also provides additional frictional damping for the pin, while 
preventing the shank portion of the pin from moving downwardly beyond the 
undersurface of the disc. In short, the floor insures that the pin is at 
all times contained and fully protected in all directions. Finally, it 
will be understood that the integral floor 100 serves to further 
strengthen the disc structure. If desired, the floor may be extended 
radially outward to provide a lower protective ledge having the same 
profile as the ledge 61 (FIG. 3). 
In the various embodiments of the invention described above, the pin has an 
enlarged head and is intended to be inserted "from the inside out". In 
accordance with one of the aspects of the present invention provision is 
made for inserting a pin "from the outside in" by avoiding an enlarged 
head and by providing latching members, for example, in the disc 
structure. Such a modified embodiment is shown in FIGS. 25 and 26, in 
which corresponding reference numerals, with addition of subscript "c", 
are employed to indicate corresponding parts. In this embodiment the head 
31c of the pin has the same diameter as the shank. The pin is engaged and 
retained by a pair of resilient latch members 101, 102 which may either be 
separate or integrally formed when molding the disc and which are oriented 
in inwardly-sprung positions engaging a reduced section 103 of the pin 
immediately under the head 31c. 
Upon insertion head first, when the head strikes the latch members 101, 102 
they are cammed outwardly and then snap into the reduced section 103 to 
hold the pin firmly in place against the effects of centrifugal force. 
When pin replacement is necessary, the old pin may be squarely severed at 
the periphery of the disc and tapped inwardly by means of a drift pin or 
the like, thereby freeing the opening for insertion of a new pin. This may 
be accomplished without having access to the top of the disc so that the 
disc need not be removed from the mower for pin replacement. 
An additional feature, shown in FIG. 26, is the use of a fully extended 
backstop 42c spaced inwardly from the edge of the disc and against which 
the entire shank and tip portions of the pin 130c can smoothly bend, with 
the tip portion being received within the periphery of the disc. In other 
words the backstop forms a recess in the edge which accommodates the 
entire backswept pin. An advantage of this, in addition to nested 
protection of the pin, is that the profile of the disc is perfectly 
circular, further minimizing the possibility of missile projection. The 
other features and advantages provided by the structures shown in FIGS. 
23-26 are equivalent to the features and advantages of the preferred 
embodiment. 
The term "stiffly resilient" has been used to characterize the tip portion 
of the cutter pin. This term has been used in order to distinguish over 
rigid and relatively non-yielding, or backed-up, cutting elements which 
have been used in the prior art and which are capable of inflicting 
serious injury upon the hand or foot of the operator. The term is also 
used to distinguish over filamentary cutting elements, for example formed 
of nylon, which are relatively limp, which tend to sag under their own 
weight, and which require centrifugal force to keep them extended and in 
cutting engagement with the grass or weeds. The present cutter pin does 
not sag under its own weight or rely upon centrifugal force to keep it in 
a cutting posture. Indeed, the pin is stiff enough so that the tip remains 
downwardly angled, to cut at or slightly below disc level in spite of the 
elevating, or leveling, effect of centrifugal force. It is nevertheless of 
sufficiently low mass, and sufficiently resilient and yielding so as to 
retreat to an out-of-the-way position upon striking an obstruction 
including the hand or foot of the operator. While the tip is capable of 
applying a painful blow to an exposed limb, severe injury of the type 
inflicted by a steel blade is avoided. 
The disc itself, while light in weight, is inherently strong and durable, 
self-reinforced by the integral bridges which overlie the notches and by 
the radial vanes. The disc is of such shape as to permit it to be readily 
molded, using nylon, delrin, or similar high impact, abrasion resistant 
plastic materials. 
While it is preferred to employ pins molded of resilient abrasion resistant 
plastics and having enlarged heads, it has been pointed out that an 
enlarged head is not essential to practicing the invention and that the 
term "head" simply refers to the inner end of the pin which is secured 
against radial dislodgement. The term "evenly spaced" as applied to the 
pins simply means spacing which is sufficiently symmetrical as not to 
result in unbalance. The term "plastic" includes tough flexible materials 
having a density characteristic of synthetic plastics in general, 
exhibiting abrasion resistance equivalent to the high performance 
synthetic materials already mentioned and capable of self-restoration to 
substantially radial position after bending at right angles around a 
curved backstop.