Prevention of slack lift chains on a man-up lift truck

• US 5,462,136 A
• Filed: 03/03/1994
• Issued: 10/31/1995
• Est. Priority Date: 03/03/1994
• Status: Expired due to Fees

• Current Assignee: The Raymond Corporation (Toyota Motor Corporation)
• Original Assignee: The Raymond Corporation (Toyota Motor Corporation)
• Inventors: Schoenmaker, John, Mathes, Gene R.
• Primary Examiner(s): NOLAND, KENNETH W
• Application Number:
  US08/205,219
• Time in Patent Office: 607 Days
• Field of Search: 187/9 R, 187/9 E, 187/140, 187/110, 187/105, 187/223, 187/229, 182/63, 182/148
• US Class Current: 187/223
• CPC Class Codes: B66F 17/003 for fork-lift trucks

Abstract

The present invention features a method and an apparatus for preventing lift chains from becoming slack in a lift truck with a "man-up" feature installed. Lift trucks are generally those vehicles that feature hydraulically controlled lift masts with a chain supported fork carriage. Operators of these lift vehicles with the remote lift and lower feature are generally lifted and lowered by these masts, known in the trade as a "man-up" condition. It is quite desirable to prevent a slack chain condition in the man-up mast position. The invention features a pressure transducer for monitoring a drop in hydraulic fluid pressure established by the flow control valve that regulates the lowering speed of the mast during the remote mode, i.e., when the fork carriage is being lowered by the operator in the platform attached to the fork carriage. The invention turns off the normally closed load-holding valve when the hydraulic pressure drops below a predetermined value, which stops the flow of hydraulic fluid back into the reservoir. This, in turn, stops the cylinder(s) from lowering and keeps the fork carriage lift chains from becoming slack.

Claims

1. A hydraulic circuit for controlling chain slackness in a lift mast, comprising:

a fluid reservoir;

a lift mast and a chain, said lift mast being capable of being raised or lowered hydraulically;

a lift cylinder operatively connected to said lift mast for raising and lowering said lift mast;

at least one fluid pump disposed between said fluid reservoir and said lift cylinder for pumping hydraulic fluid from said reservoir to said lift cylinder;

a fluid flow valve disposed between said reservoir and said at least one fluid pump for regulating fluid flow to said lift cylinder, hence controlling the raising and lowering of said lift mast;

a load-holding valve disposed between said lift cylinder and said reservoir;

a transducer operatively connected to said fluid flow valve for sensing fluid pressure in said fluid flow valve, and, in response to a given low-pressure level, generating a signal; and

a controller operatively connected to said transducer and said load-holding valve, said controller blocking fluid flow through said load-holding valve in response to said signal, whereby said lift mast will be prevented from lowering and said chain from slackening.

2. The hydraulic circuit in accordance with claim 1, wherein said lift mast is operative in a remote mode, wherein the raising and lowering of said lift mast are the only possible operative functions.

3. The hydraulic circuit in accordance with claim 1, wherein said controller is programmable to prevent said chain from slackening.

4. The hydraulic circuit in accordance with claim 1, in combination with a reach truck.

5. A hydraulic system for controlling chain slackness in a lift mast, comprising:

a fluid reservoir;

a lift mast and a chain, said lift mast being capable of being raised or lowered hydraulically;

a lift cylinder operatively connected to said lift mast for raising and lowering said lift mast;

a fluid flow valve disposed between said reservoir and said lift cylinder for regulating fluid flow to said lift cylinder, hence controlling the raising and lowering of said lift mast;

a load-holding valve disposed between said lift cylinder and said reservoir;

a transducer operatively connected to said fluid flow valve for sensing fluid pressure therein and, in response to a given low-pressure level, generating a signal; and

a controller operatively connected to said transducer and said load-holding valve, said controller blocking fluid flow through said load-holding valve in response to said signal, whereby said lift mast will be prevented from lowering, and hence preventing a slackening of said chain.

6. The hydraulic system in accordance with claim 5, in combination with a reach truck.

7. The hydraulic system in accordance with claim 5, wherein said lift mast is operative in a remote mode, wherein the raising and lowering of said lift mast are the only possible operative functions.

8. The hydraulic system in accordance with claim 5, wherein said controller is programmable.