Other Factors to Consider

3.1 System Operating Characteristics

Typically, when designing a liftgate system there will be several performance guidelines. These may include:

Cold closing and opening efforts
Room temperature closing and opening efforts
Hot closing and opening efforts
Self-rise and self-close angle
Hump
Dampening

All these characteristics have compromises because many will directly and adversely affect another characteristic. When designing a liftgate system it is important to prioritize the operational targets. Usually the first requirement is the cold hold open effort. Typically this is set to 3.0 pounds at -30C for an automotive liftgate or hatch. Once this is done most of the remaining characteristics are set. However, some characteristics can be improved by adjusting the mounting positions of the gas spring and/or the handle positions. Another element that is sometimes overlooked is the opening angle needed to satisfy the needs of the tallest and shortest operators. Generally, it is beneficial to make the opening angle the smallest possible. This will make the designers job of reducing any of the undesirable characteristic easier.

3.1.1 Opening and Closing Efforts

For the most part, opening and closing efforts are dictated by one thing: liftgate weight. The lighter the liftgate the easier the opening and closing efforts will be at all temperatures. Once a cold hold open has been set for a particular liftgate design and gas spring mounting, little can be done to improve any of the owner effort characteristics. Even changing the prop geometry may have limited improvement if the liftgate is very heavy. Currently, opening and closing efforts for an automotive liftgate above 50 pounds total weight should be 12 to 15 pounds to open and close. Acceptable efforts would be in the 15 to 18 pound range. Any efforts above 18 pounds may result in a high rate of owner dissatisfaction.

3.1.2 Self-Rise Angles

The self-rise angle is the angle at which the gas spring will lift the door without any assistance from the operator. For most systems this will take place between 10° and 30° from the full closed position. This angle will become greater as the temperature falls from ambient and will be smaller as the temperature rises. If the designer tries to make the self-rise angle small it will tend to make the closing efforts high. This is because in order for the system to have a small self-rise angle the output force in the compressed position will have to raise. Raising the compressed force tends to raise the extended output force at some proportional rate. Conversely, if the designer wants low closing efforts, he may cause the self-rise angle to be large.

Self-close is related to self-rise. Self-close is the angle at which the door will close without any assistance from the operator. The only reason these two angles are not exactly the same is due to friction. Earlier in this document we discussed friction internal to the gas spring. This is one of the sources of friction. Another source is the friction in the hinge or hinge system and the gas spring connectors. In simple pivot systems generally the friction from the hinge is small, providing there is no binding within the hinge. However, in multi-link hinges the friction from the hinge system can be very significant. In these type of systems the bushings in each revolute joint become important design considerations when developing the hinge system. Lastly, there is the friction from the gas spring connectors. These generally input very little friction into the system, but they are a consideration if the system is exhibiting a large angle difference in the self-rise and self-close angles.

Figure 21

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