FIELD OF INVENTION
This invention relates to a shift fork which forms part of a gear shifting mechanism in a manual transmission gear box of an agricultural tractor.
BACK GROUND OF INVENTION
A shift fork used to move a coupling sleeve in an axial direction when a transmission gear shift is performed, and engaging part which engages with the coupling sleeve is formed at the end of the fork main body. Conventionally by high precision injection molding of polymer material, it is possible to mold a fork main body and engaging part in one-piece integral unit, so as to decrease costs and achieve a light weight construction.
The engaging part engages on the groove of the rotating coupling sleeve. Generally flat extra strong engaging surfaces are formed on the tips of the engaging part. Attempts are made to strengthen the engaging part, to prevent deflection.
OBJECTS OF THE INVENTION
It is one of the objects to provide a lasting solution by eliminating the use of metal
and substitute the same with the synthetic components. According to the invention,
this object is achieved in shift fork of the kind referred to the invention. t
It is therefore the other object of the invention to provide a low cost, light weight shift fork for a transmission having lasting life & zero post processing.
The another object of the invention is to make the fork as a recyclable, self- lubricating & non-rusting component.
DESCRIPTION OF PRIOR ART
A conventional transmission unit is gear box, the gear box has a housing, a gear shaft rotatably mounted on the housing, a plurality of gears rotatably mounted on the shaft, and connecting member movable along the shaft for individually connecting the gears to the shaft.
The connecting member can be a shift key located in the longitudinal groove in the shaft and having a lug extending upwardly to engage recesses in the individual gears to mechanically connect the gear to the shaft to rotate therewith. The shift key has a projection at the end opposite the lug which is engaged by a collar positioned around the shaft. A shift fork is employed for moving the collar and shift key along the shaft to individually engage the gears.
The detent and shift arm may also combined as one piece, thus simplifying a shift mechanism.
Also a typical motor vehicle transmission includes a plurality of longitudinally extending shifter rails upon which shift forks are slideably mounted. Gears are individually associated with each fork and move therewith.
A shifter lever extends downwardly from the cover of transmission and terminates at a point slightly above the shifter rails so as to allow the lower end of the shifter lever to selectively engage one the shift forks, as by capture of the lower end of the shift lever between the exposed sides of an offset arm extending upwardly from each of the shift forks. Each of the shift forks may be selectively positioned at a predetermined point along its associated shifter rail so as to place its associated gear in a desired relation to other gears. Movement of the shift lever is controlled by the operator of the vehicle.
When repositioning of one of the shift forks, it is desirable to prevent the movement of other forks, as only one gear should be engaged at one time. A gate is typically provided to prevent unwanted movement. Such a gate may typically comprise a flat plate mounted on gate rails for transverse movement in a plane above and parallel to the plane of shifter rails. The gate rails extend in a direction perpendicular to that of shifter rails. The shift lever extends through a slot in the gate and effect movement of the gate when moved in the direction perpendicular to the direction of the shifter rails. Movement of lever in a direction parallel to the shifter rails does not cause movement of gate. The gate is provided with downwardly extending tabs to engaged at least one shift fork to prevent movement thereof when the shift lever engages another fork which is to be repositioned. The shift lever gate may be returned to .„ Jtral when gears are disengaged. Springs are typically provided to urge the shift lever and gate to neutral position.
However, due to multiple components, it was developed later into a unitary construction wherein axis of rotation of shift lever intersects that of a rotatable selector gate. Further shift lever is contained within selector gate and cooperates therewith to shift the shift fork of a conventional transmission mechanism.
A plurality of conventional shifter rails extend longitudinally and plurality of shift forks are suspended, one from each of the rails and are slidabely mounted thereon, by circular sleeve portions, which sleeve is an integral part of each of the forks.
A forked portion of each shift forks extends upwardly of each sleeve. Selective positioning of each of the shift forks on respective rails, serves to shift the gears on the transmission in a predetermined conventional manner.
In normal automotive vehicle transmission, when a driver operates a shift lever to effect a speed ratio change, the displacement of the shift lever is transmitted through the link mechanism to a shift fork engaging a coupling sleeve, rotating with a drive shaft, thereby axially moving the coupling sleeve into engagement with a gear rotating freely on the drive shaft so as to lock the gear to the drive shaft. Since speed ratio changes are frequently effected during driving, the shift fork and coupling sleeve are subject to great wear at their contacting portions. This causes a reduction of displacement of the coupling sleeve resulting in failure to effect certain and smooth speed ratio changes. In order to eliminate such difficulties, attempts have been made to harden the surface of shaft fork and to cover the contacting portion of the shift fork with coupling sleeve with metal having a high anti-wear property. However, these attempts cannot prevent the wear of coupling sleeves although they can prevent the wear of shift fork. Further since the contacting portion of the shift fork is smaller than the contacting portion of the coupling sleeve, the anti-wear material becomes worn out in a relatively short time. Thereafter attempts were made such that coupling sleeve has its portions contacting the shift fork covered with wear- resistance material having high self-lubricating and anti-wear properties.
US 1,482,014 is an application relating to shift fork. It speaks of separate friction shoes adaptable on the fork, so that control fork can be made of any sufficiently
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shifts. As such some of the metals or materials suitable for one may not be suitable for the other.
DESCRIPTION OF THE DRAWINGS
Fig l illustrates the prior art of metal shift fork for tractor transmission gear box, illustrating the assembly of shift fork to the rail.
Fig 2 illustrates the front and back view as a whole metal shift fork as per prior art. Fig 3 illustrates the invention with polymer gear shift fork for tractor transmission gear box, illustrating the assembly of shift fork to the rail.
Fig 4 illustrates the front and back view as a whole polymer gear shift fork as per invention.
Reference numerals in the enclosed drawings are as follows:-
1. Shift fork.
2. Grub Screw.
3. Rail.
4. Spring Dowel, and
n. Fork body
12. Engaging portion
13. Semi circular first arc portion
14. Semi circular second arc portion
15. Terminal tips
16. Cylinder hollow boss
17. Bore holes
18 &19 Groove channels
DESCRIPTION OF THE INVENTION
Referring to Figure 3, a shift fork for manual transmission is made by high precision injection molding of Nylon 66 material having a main body(n), an engaging part(is) at one end and a cylindrical hollow boss(i6) at the other end to connect with shift rai(3).
The shift fork(i) has at its free ends of engaging parts(is), engaging surfaces adapted to be received in an annular groove(i8,19) of a shift element. The ends itself being molded with raised edges thereby eliminating the need for any specific insert or shoe for enhancing contacts life.
The raised edges of engaging end have a depth 'A', which is greater than the depth 'B' of engaging part. By virtue of this predetermined difference in depth, each end is raised edge that extends beyond the fork clearance surface.
The improved shift fork assembly of this invention is capable for adaptation for use in combination with existing transmission mechanism.
The type of fork envisaged comprises of the two arms making up the engaging part, which goes around a periphery of a coupling sleeve that is rotated during operation of the gear box. The shift fork is sufficiently resistant to absorb the synchronization stresses which is transmitted during operation of gear box. The material has a low- efficient of friction in the range 0.2 - 0.3 and a good resistance to wear.
The use of glass in nylon material also ensures that the fork has good mechanical properties. Further the materials ensure reduction in weight of shift fork and are very cheap. Even when metals like casting iron or steel are used, due to not too good mechanical resistance to absorb synchronization stresses, the end surfaces are treated with expensive high partly chromium or molybdenum.
The ends may be so shapely molded to fit on the end shoes, prior to mounting of shift fork in a gear box such that said shoes do not come off during operation. The use of synthetic plastic material makes it convenient and easy to obtain such shape and configuration without involving heavy additional cost or labour.
The proposed gear shift fork is made of nylon by injection molding, wherein in the proposed structure, the performance of shift fork will improve, this being due to precision molding made possible as per desired design. The shift fork as in the invention, may appear to be a very simple structure, which however produces a practically a very useful effect on the performance and life.
It is possible to now to provide a novel and improved gear shift fork for automotive transmissions which will perform well and which is suited for adapting to conventional transmission mechanism. It is attempted to do away with a shift fork which in prior art has a body made of aluminum, cast iron or steel, which may crack or bend over a period. Attempts have been made repeatedly with metal body and strengthened anti-wear contact tips, however such attempts have not been totally
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satisfactory from the issue of reliability, cost of manufacturing and simplicity of construction.
As per the invention it also attempted to reduce the weight of shift fork wherein if a typical conventional metal shift fork is 440 gms, an equivalent shift fork will only weigh 140 gms. There is approximately 70% weight reduction. To improve the flexural rigidity of the nylon 66 material 30% glass filling is done.
The conventionally made forged, steel or investment casting and machined to fine dimension, generates scrap in forging and machining of very high volume, which leads to low yield. In the proposed invention, there shall be zero scrap.
In the conventional shift fork, a typical conventional metal shift fork is assembled with rail by means of grub screws locking, where as in the proposed invention spring dowel sleeve is used for assembling with rail.
The material used herein is cheaper, lighter, non-rusting in nature, recyclable & self- lubricating material. The integral unit & modified locking device lead to quicker assembly. The injection molding enables zero post processing and zero scrap.

WE CLAIM
l. A gear shift fork (1) made of synthetic resin material disposed upon, supported and engaging with a shift rail (3) of a transmission, said shift fork(i) comprising a fork body(n) and a cylindrical hollow boss(i6) having interior surface which engages with said rail(3) and integrally molded with said fork body(n), said body extending into engaging portion(i5) to form a generally U shaped fork, the improvement in said gear shift fork mechanism comprising :-
said fork body (11) and engaging portion (15) integrally molded therewith, said fork body(ii) and engaging portion(i5) are of same material,
said fork body having 'I' section stem member.
said stem member at one end extending laterally onwardly to form a semi¬circular first arc portion (12), with uniform predetermined generally semi¬circular space formed (13) there between,
said first arc portion(i2) along a slanting wall tapering to form a stepped semi-circular second arc portion (14),
said second arc portion(i4) having two solid terminal tips (15) at the two extreme ends of the arc, forcing a sturdy engaging surfaces,
said stem member at the other second end having a cylindrical hollow boss (16) defined in the interior surface of the shift fork,
said second end having a cylindrical boss(i6) having a through hole (17) for assembling spring dowel sleeve(4) defining apertures,
a spring dowel sleeve (4) adapted for arrangement across the said cavity through the holes as fastener member adapted for restraining the fork with the shift rail(3), and
said stem having plurality of groove channels (18, 19) for predetermined length in the direction along the horizontal axial length of the body, on two opposite surfaces perpendicular to the arc to form T section.