2.3 Push Up Design

The most preferable application type is what AVM terms push up. The most identifiable trait of this type of design is that the end of the gas spring that is lowest in the closed position stays in the lowest position throughout the opening cycle. When using this type of system it is also preferable to mount the gas spring in a shaft pointing down orientation. This orientation insures proper lubrication of the seal system and provides a more repeatable dampening characteristic. An example of this type of application in a hatch system is illustrated in Figure 15.

Figure 15

Typically, this type of system is also identifiable by the mounting position of the hatch or moving element located closer to the hinge than the body or fixed element mounting position.

The biggest drawback to using this type of system is the difficulty in providing the structural strength required for the system. This type of system requires a stronger structure around the liftgate mounting. This is due to the increased cantilever of the liftgate from the gas prop support.

2.3.1 Crossover - Self-Rise - Self-Close

Crossover is the point in the opening cycle where the gas prop takes over all the lifting action (self-rise). At this point no further assistance is required by the operator for the door to reach the fully open position. There is a corresponding crossover position for the closing cycle where the door will fall to the closed position with no operator assistance. The actual angle at which these two events occur are usually separated by a few degrees. This separation is due to friction in the gas spring internal components and connectors and with the hinge. For example: if we consider the closed position to be 0°, then the crossover for the self-rise position will occur typically around 25° compared to about 20° for self-close.

2.3.2 Hump

Hump is defined in all systems as the difference between the force required to begin closing the liftgate and the maximum force required to close the liftgate at any point in the closing cycle. The hump characteristic is fairly easy to visualize in a push up system. (Figures 16 and 17). The hump generally occurs at the point where the gas prop has the greatest moment arm. This can be adjusted by changing the geometry to insure that the prop has the greatest moment arm in the full open position.

Figure 16

Figure 17

2.3.3 Dampening

Consistent dampening is a major advantage of shaft down orientation. With this type of system the oil is always pooled at the bottom of the tube. During the opening cycle the piston moves down to the oil to begin the oil dampening consistently at the same location during each cycle.

Figure 18

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