A conventional gearshift in a gearbox of a motor vehicle (e.g., automobile) for rough terrain is shown in FIG. 1. The gearshift comprises an output shaft 11, a shaft for gearshift fork 12, and a speed changing case 13. The output shaft 11 comprises a first sliding block 111 and a second sliding block 112. A first groove 113 is provided on the first sliding block 111 and a second groove 114 is provided on the second sliding block 112 respectively. The shaft for gearshift fork 12 comprises a first gearshift fork 121 having one end fitted in the first groove 113, and a second gearshift fork 122 having one end fitted in the second groove 114 respectively. A first positioning peg 123 is formed at the other end of the first gearshift fork 121 and a second positioning peg 124 is formed at the other end of the second gearshift fork 122 respectively. A first trough 131 and a second trough 132 are formed on an outer surface of the speed changing case 13 and are disposed corresponding to the first positioning peg 123 and the second positioning peg 124 respectively. Thus, the first positioning peg 123 is adapted to fit in the first trough 131 and the second positioning peg 124 is adapted to fit in the second trough 132 respectively. By configuring as above, the shaft for gearshift fork 12 may slide in response to sliding the speed changing case 13. As an end, the purpose of shifting gears is achieved.
For causing the gearshift to smoothly change from a reverse gear (R) to a drive gear (H) or from a drive gear (H) to a reverse gear (R), a spring 14 is put on a cylindrical portion projecting from one side of the first gearshift fork 121. A groove machining process is performed on the first trough 131 of the speed changing case 13 as illustrated in FIG. 2. As shown FIGS. 1 and 2, dash line 133 indicates a state of the first trough 131 prior to the groove machining process, and solid line 134 indicates a state of an additional groove being formed in the first trough 131 by the groove machining process. A portion of the first trough 131 indicated by the solid line 134 is straight. Also, the spring 14 is adapted to exert an elastic force. As such, a change from a reverse gear (R) to a drive gear (H) or from a drive gear (H) to a reverse gear (R) is made quick and smooth via an overdrive gear (L). As an end, shifting gears is facilitated.
While the above configuration is able to facilitate shifting gears, it is disadvantageous for requiring the additional groove machining process in forming the first trough 131 since it can complicate the process, increase the manufacturing time, and greatly increase the manufacturing cost. Further, the spring 14 is not engaged with the main body of the first gearshift fork 121. Furthermore, an elastic force exerted by the spring 14 may hinder a mounting of both the spring 14 and the first gearshift fork 121 on the shaft for gearshift fork 12, resulting in time consuming of the assembly process. For eliminating the additional groove machining process in forming the first trough 131 and facilitating the mounting of both the spring 14 on the first gearshift fork 121 of the shaft for gearshift fork 12, a gearshift of a motor vehicle for rough terrain is developed by some manufacturers of the art as shown in FIG. 1. The gearshift likewise comprises an output shaft 11, a shaft for gearshift fork 12, and a speed changing case 13. The shaft for gearshift fork 12 comprises a first gearshift fork 121 and a second gearshift fork 122 both of them adapted to move together with the speed changing case 13. Further, the output shaft 11 comprises a first sliding block 111 assembled with the first gearshift fork 121, and a second sliding block 112 assembled with the second gearshift fork 122 so as to facilitate shifting gears.
Referring to FIG. 3, the first gearshift fork 121 is put on an outer surface of a sleeve 120 at one end. On the outer surface of the sleeve 120 from the first gearshift fork 121 to the other end of the sleeve 120, there are provided a guide block 125, a spring 126, a washer 127, and a positioning member 128 sequentially. The guide block 125 is extended to fit in the first trough 131 on the speed changing case 13 such that both the guide block 125 and the first trough 131 are adapted to slide together. The additional groove machining process in forming the first trough 131 is thus eliminated due to the conformed shape of the guide block 125 opposite the first gearshift fork 121. Further, positioning member 128 is fastened at the other end of the sleeve 120. As such, the guide block 125, the spring 126, and the washer 127 are securely put on the sleeve 120. Thus, an elastic force exerted by the spring 126 is prohibited from hindering a mounting of the first gearshift fork 121 on the shaft for gearshift fork 12, resulting in a facilitation of the assembly process with time being saved.
Referring to FIG 4, for further facilitating the gearshift a positioning pin 129 is provided on the first gearshift fork 121 facing the guide block 125. The positioning pin 129 is also located on the sleeve 120 proximate to the guide block 125. Referring to FIG 5, a state of the speed changing case 13 in a drive gear (H) is shown. The spring 126 is compressed by the guide block 125 when the first sliding block 111 does not travel to enter into the groove 151 of a drive gear 15. As such, the guide block 125 slides over the positioning pin 129 until the first sliding block 111 enters into the groove 151 of the drive gear 15. To the contrary, the spring 126 is not compressed by the guide block 125 when the first sliding block 111 enters into the groove 151 of the drive gear 15 smoothly. By providing the positioning pin 129 as above, it is possible to configure the gearshift to smoothly shift gears from the overdrive gear (L) to the reverse gear (R) or vice versa.
However, for the gearshift shifting gears requires waiting the guide block 125 to slide over the positioning pin 129 until the first sliding block 111 smoothly enters into the groove 151 of the drive gear 15. The waiting is a relatively long time. Further, the first gearshift fork 121 and the guide block 125 move due to the provision of the positioning pin 129. However, these components put on the sleeve 120 are prone to loosening after a period time of use. As a result, the components tend to be damaged. Furthermore, a smooth speed change is not possible because force exerted thereon is unbalanced. And in turn, it may cause jamming. All of the above drawbacks may bring great inconvenience to users. Thus, continuing improvements in the exploitation of gearshift fork for a motor vehicle are constantly being sought.