8.1 GENERAL GUIDELINES

Kinematics is the term applied to the design and analysis of those parts used to retract and extend the gear, with particular attention to the determination of the geometry in the retracted and extended positions. Basic guidelines are as follows:

1. Start with a geometric layout, but replace this with a mathematical analysis as soon as possible.

2. Ensure that satisfactory moment arms are provided throughout the travel.

3. Use the simplest possible kinematics.

4. Approximate the actuator "dead length" (see Fig. 8.1) in the preliminary design layout. The following are suggested:

Figure 8.1: Actuator dead length.

The lower and higher values generally apply to smaller/larger diameter actuators, respectively. For instance, the Lockheed JetStar side brace actuator, with one internal lock, has a dead length of 10.5 in.

The above values include an estimated 1 in. of length for the actuator end fitting. This can be deducted if a trunnion mount is used (such as shown in Fig. 8.2), but on a hydraulic or pneumatic actuator, this type of mount is relatively expensive. It is, however, the optimum type for ball screw actuators, in which the trunnion is at the ball nut. Offset mounts (Fig. 8.3) should be avoided, since they cause undesirable stresses and deflections in the actuator. It should also be recognized that these dead lengths are ultimately dependent upon the detail design of the actuator to meet specific conditions and may vary somewhat from the above suggestions. Other factors to keep in mind are:

Figure 8.2: Trunnion-mounted actuator.