GB2287694A

GB2287694A - Fork lift trucks

Info

Publication number: GB2287694A
Application number: GB9505768A
Authority: GB
Inventors: Frederick George Wilson
Original assignee: Individual
Current assignee: Individual
Priority date: 1994-03-24
Filing date: 1995-03-22
Publication date: 1995-09-27
Anticipated expiration: 2015-03-22
Also published as: GB2287694B; GB9405861D0; GB9505768D0; DE19510328A1

Description

2287694 FORKLIFT TRUCKS This invention relates to forklift trucks, in

particular but not exclusively demountable forklift trucks.

Conventionally, the mast of a forklift truck comprises an upright outer frame mounted for rotation about a horizontal axis at the front of the forklift truck chassis for tilting of the mast, and an inner frame slidably mounted by rollers on the outer frame for raising and lowering relative to the outer frame. Finally, a roller-mounted fork carrier is slidably mounted on the inner frame for raising and lowering relative to the inner frame.

Power means, usually a hydraulic ram or rams, is provided to raise or lower the inner frame relative to the outer frame.

is Chains are secured to the chassis of the forklift truck at one end and passed over the top of the inner frame and attached to the fork carrier at the other end. In service, by raising the inner frame, the fork carrier is raised by twice the amount of the movement of the inner frame.

In a demountable forklift truck, means are provided to attach the forks to the rear of the vehicle and when thus secured to the vehicle the power means are used to lift the chassis of the forklift truck above the ground at the rear of the vehicle for travel.

This lifting operation is made possible by having the lifting hydraulic ram of the double acting type and providing, in addition to the chains passing over the inner frame, a further chain or chains attached at one end to the underside of the fork carrier and passing around the bottom of the inner frame and secured to the chassis.

Demountable forklift trucks are generally only required to have a lift height sufficient to lift cargo from the floor of a vehicle to the ground and vice versa, typically over a vertical height of 1.5 metres, and it is sufficient therefore that a demountable forklift has a lift height approximate to this.

The object of the hr. ention is to provide a forklift truck having a mast which can be constructed at relatively lower cost and of lighter weight than the conventional kind described above.

According to the present invention a forklift track has a mast comprising a single upright frame mounted at the front of the forklift truck chassis, a fork carrier slidably mounted on the frame for raising and lowering relative to the frame, and two hydraulic rams whose cylinders are secured together and whose piston rods extend in opposite directions, the end of one piston rod being secured to the fork carrier and the end of the other piston rod being secured to the frame or to the chassis.

The advantage of the invention is that because only a single frame is used which is directly mounted to the front of the forklift chassis (that is to say, is it doesn't slide within another frame), chains are not required nor are the rollers associated with the normally provided inner sliding frame, so there is the potential of both a significant weight saving and a saving on manufacturing cost. On the other hand, the provision of the two hydraulic rams secured together with their piston rods facing in opposite directions permits the maximum height of lift within the single frame to be obtained.

An embodiment of the invention will now be described, by way of example, with reference to the accompanying drawings, in which:

Figure 1 is a perspective view of a mast for a forklift truck with the fork carrier at its highest position on the mast; Figure 2 is a perspective view of a mast of figure 1 with the fork carrier at its lowest position on the mast; and Figure 3 is a schematic cross-section of the double ram arrangement of figures 1 and 2.

Referring to figures 1 and 2, a forklift truck has a mast comprising an open, substantially rectangular steel frame 10 having substantially parallel t side members 11 and 12 joined at the top and bottom respectively by cross members 13 and 14. The frame 10 is mounted upright at the front of the chassis of a forklift truck, and is capable of slight rearward tilting by rotation about a horizontal axis at the front of the chassis. The forklift truck itself is not shown, but it, and the manner of mounting the frame 10 onto the chassis, may be conventional.

A generally rectangular steel fork carrier 15 comprises two upright elongate plates 16 and 17 connected by horizontal cross beams 18. The plates 16 and 17 fit between and are roller-mounted on the respective side members 11 and 12 of the frame 10 so that the fork carrier 15 as a whole is slidably mounted on the frame 10 for raising and lowering relative to the frame. Figure 1 shows the fork carrier 15 at its highest position on the frame 10, and Figure 2 shows the fork carrier at its lowest position on the mast. Forks 19 are mounted on the cross beams 18 of the fork carrier.

Power for moving the fork carrier 15 up and down the frame 10 is provided by two hydraulic rams 20 and 21 whose cylinders 22 and 23 respectively are secured together and whose piston rods 24 and 25 respectively extend in opposite directions substantially parallel to the side members 11 and 12 of the frame 10. The end of the piston rod 24 is secured to the fork carrier 15 and the end of the other piston rod 25 is secured to the cross member 14 of the frame 10. Alternatively, the end of the piston rod 25 may be secured to the chassis of the forklift truck. In this embodiment the hydraulic rams are double acting.

The upper cross beam 18 of the fork carrier has an upward generally inverted U-shaped extension 29 which embraces the top portion of the cylinder 22 when the fork carrier 15 is at its lowest position in the frame 10 (figure 2) and projects above the top cross member 13 of the frame 10 when the fork carrier 15 is at its highest position in the frame (figure 1), and the end of the piston rod 24 is secured to top of the extension 29. This arrangement permits a fall range of movement of the fork carrier 15 on the frame 10.

By extending the hydraulic rams 20 and 21 the fork carrier 15 is pushed to the top of the mast, figure 1. Further, because the rams are double acting, the hydraulic rams 20 and 21 can be retracted to pull the fork carrier 15 to the bottom of the mast, figure 2. Thus, by fixing the forks 19 to the rear of a vehicle at the level of the vehicle platform, retracting the rams 20 and 21 will raise the forklift truck may be raised from the ground for travelling.

Referring now to figure 3, the hydraulic pressure to both of the rams 20 and 21 is transmitted via the piston 25 of the downwardly facing ram 21.

The piston rod 25 is hollow and has a internal pipe 30 extending from an inlet connection 33 at the lower end of the piston rod 25 through the piston 31 to the full bore side 32 of the cylinder 23. The lower end piston rod 25 has another inlet connection 34 to the interior of the piston rod 25 surrounding the pipe 30, and an aperture 35 is provided in the wall of the 15 hollow piston rod 25 close to the piston 31. An external pipe 36 connects the full bore side 32 of the cylinder 23 with the full bore side 37 of the cylinder 22, and a further external pipe 38 connects the annular side 39 of the cylinder 23 with the annular side 40 of the cylinder 22.

By applying hydraulic fluid under pressure to the inlet connection 33 the full bore sides 32 and 37 of both cylinders are pressurised, the cylinder 23 by the pipe 30 and the cylinder 22 further by the pipe 36, so that both rams 20 and 21 are extended. During this, the hydraulic fluid in the annular sides 39 and 40 of the cylinders is exhausted through the inlet connection 34 via the pipe 38 and aperture 35.

Conversely, when hydraulic fluid under pressure is applied to the inlet connection 34 the annular sides 39 and 40 of the cylinders are pressurised via the aperture 35 and pipe 38 so that the rams 20 and 21 are retracted, and fluid in the full bore sides 32 and 37 of the cylinders is exhausted through the inlet connection 33 via the pipes 30 and 36.

The hydraulic fluid pressure for operating the rams may be derived in the conventional manner for forklift trucks.

MC

Claims

1. A forklift truck having a mast comprising a single upright frame mounted at the front of the forklift truck chassis, a fork carrier slidably mounted on the frame for raising and lowering relative to the frame, and two hydraulic rams whose cylinders are secured together and whose piston rods extend in opposite directions, the end of one piston rod being secured to the fork carrier and the end of the other piston rod being secured to the frame or to the chassis.

2. A forklift truck as claimed in claim 1, wherein the fork carrier has an upward extension which projects above the top of the frame when the fork carrier is at its highest position in the frame, and wherein the end of the one piston rod is secured to top of the extension.

3. A forklift truck as claimed in claim 1 or 2, wherein the hydraulic rams are double acting.

4. A forklift truck as claimed in claim 3, wherein hydraulic pressure to both rams is transmitted through the said other piston rod.

5. A forklift truck having a mast substantially as described herein with reference to the accompanying drawings.