Cover lift mechanism

1. A lift mechanism for use in manipulating trench covers, comprising:

an axially extending housing;

a cam located adjacent a first end of the housing, the cam being capable of moving between a first cam position and a second cam position;

a latch located adjacent a second end of the housing, the latch being operatively associated with the cam and capable of rotational movement between a latch retracted position when the cam is in the first cam position and a latch actuated position when the cam is in the second cam position;

whereby the lift mechanism may be inserted into or removed from a cover aperture when the latch is in the latch retracted position, and the lift mechanism may engage and support the cover when the latch is in the latch actuated position; and

wherein the housing at least partially encloses an actuation piston connected to the cam, the piston being capable of limited axial movement relative to the housing between first and second piston positions, whereby when in the first piston position the cam is in the first cam position and the latch is in the latch retracted position, and in the second piston position the cam is in the second cam position and the latch is in the latch actuated position.

2. The lift mechanism of claim 1 , further comprising a latch retraction spring and a latch actuation spring, the springs connecting the piston to the latch and having different spring rates, wherein the latch retraction spring controls the position of the latch when the cam is in the first cam position and the latch actuation spring controls the position of the latch when the cam is in the second cam position.

3. The lift mechanism of claim 2 , wherein the springs have different body lengths.

4. The lift mechanism of claim 2 , wherein the piston is connected to the latch by a combination of springs and cables.

5. The lift mechanism of claim 1 , wherein in the second cam position the cam is axially aligned with the housing.

6. The lift mechanism of claim 1 , further comprising a connector associated with the cam, facilitating linking of the mechanism to a device for generating a force on the connector sufficient to remove the cover.

7. A lift mechanism for use in raising trench covers, comprising:

an axially extending housing;

a cam located adjacent a first end of the housing, the cam being connected to the actuation piston and capable of moving between a first cam position and a second cam position;

a latch located adjacent a second end of the housing, the latch being operative associated with the cam and capable of rotational movement between a latch retracted position when the cam is in the first cam position, and a latch actuated position when the cam is in the second cam position; and

latch driving linkage connecting the cam to the latch;

whereby the lift mechanism may be inserted into or removed from a cover aperture when the latch is in the latch retracted position, and the lift mechanism may engage and support the cover when the latch is in the latch actuated position.

8. The lift mechanism of claim 7 , wherein the latch driving linkage includes an actuation piston at least partially enclosed by the housing and connected to the cam, the piston being capable of limited axial movement relative to the housing.

9. The lift mechanism of claim 7 , wherein the latch driving linkage includes two springs of having different spring rates.

10. A method for using a lift mechanism to raise ground trench covers, comprising the steps of:

positioning an axially extending housing over an aperture of a cover to be raised;

causing a cam located adjacent a first end of the housing to be placed in a first cam position, resulting in a latch located at a second end of the housing to be placed in a latch retracted position in which the latch is substantially axially aligned with the housing;

inserting the latch within the aperture of the cover;

causing the cam to be placed in a second position cam, resulting in the latch to be placed in a latch actuated position in which the latch is substantially out of axial alignment with the housing; and

raising the cover by generating a lifting a force on the mechanism.

11. The method for using the lift mechanism of claim 10 , wherein the first cam position is one in which the cam is not in axial alignment with the housing and the second cam position is one in which the cam is in substantial axial alignment with the housing.