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Mechanisms – Pushrod

A pushrod is a rigid structural link used in flight control mechanisms. A pushrod typically connects to a bellcrank and/or forms the coupler link of a four bar linkage assembly. A generic pushod is shown in Figure 1. The pushrod consists of a center body and two adjustable rod ends. The rod ends are adjustment to support rigging of all or part of a mechanism. Since the rod ends are threaded and screw into the center body, a means for ensuring sufficient thread engagement is required. Quite often this is done through a witness hole (see Figure 1) where the threads must be engaged up to the witness hole as a minimum. Thus the witness hole provides a means to verify proper engagement. The rod ends are held in place through various locking means that include a jam nut or tabbed washer or safety wire or some combination of these. It is critical to ensure the rod ends will not move once adjusted and locked into place.

Figure 1 Generic Pushrod

Pushrods are commonly made out of steel but also can be manufactured out of aluminum or composite material. The rod ends are generally manufactured to a MS specification. Spherical bearings are located in each rodend. The threads, which are defined by the MS specifications, will usually be small pitch to ensure small adjustments are possible – rod ends can only be adjusted in increments of 180 degrees.

Figure 2 shows further detail of the eyebolt connection to the control rod. Figure 2 shows how the eyebolt threads into control rod and the location of jamnut. Once the eyebolt is threaded into the control rod to the witness hole, the jamnut is tightened on the control rod and then safety wired to the locking tab. The locking tab is held in place using serrated washers. In some pushrods a bent tab washer is used where the washer has tabs that can be bent along the flat spot of the jamnut and along a flat spot on the control rod (this configuration is now shown in the figures). Note that the eyebolt can be positioned at any angle and then locked into place at the desired angle. Hence the eyebolts at each end of the pushrod can be at different angles (e.g., one straight and the other rotated 90 degrees). Also, the pushrod length is adjusted on one end by rotating the eyebolt 180 degrees. The change in length will be number of turns x the thread pitch. All pushrods will have a spherical bearing in each eyebolt. The spherical bearing accommodates installation tolerances and ensures that the pushrod load will only be axial.

Figure 2 Eyebolt / Control Rod Connection

Figure 3 shows a more complicated eyebolt to control rod connection. In this arrangement, the eyebolt threads into a vernier sleeve, which is then threaded into the control rod. There are two jam nuts and two sets of serrated washers now because of the two threaded attachments. The jam nut and vernier sleeve could be locked wired together or locked wire to a locking tab (if installed), or even wired to the locking tab shown. This arrangement with the vernier sleeve would provide for finer adjustments in the pushrod length. The arrangement could also be used to lengthen a pushrod if a particular pushrod was a little short for a given application.

Figure 3 Eyebolt / Control Rod Connection

Besides have the proper length, the primary design criteria for a pushrod is strength. Pushrods carry an axial load only and must be sized for the expected maximum loads expected in service. Traditionally, pushrods have been manufactured out of steel, both the control rod and eyebolt. In low load applications, aluminum pushrods have been used. More recently, composite control rods are being manufactured and used in some applications. Composite control rods should be thoroughly tested in a given application to prove out the strength and durability of the composite material and the threaded connection between the eyebolt threads and control rod threads. Means of locking – jamnuts, safety wire, locking tab, bent tab washers, etc. – should always be included in pushrod. Another means of locking protection would be to use LH threads on one end of the control rod and RH threads on the other end of the control rod. The torque applied to a jamnut should never be considered a locking means.

Secondary design considerations are adjustability and the locking method for the threaded rod end. In some applications, a very fine adjustment of the pushrod length is required. Fine adjustments can be done by using a small thread pitch, using a vernier sleeve arrangement as shown in Figure 3, or by adjusting both rod ends simultaneously.

Spherical bearings can tolerate some amount of mis-alignment (3-5 degrees) during installation. If the mis-alignment capability of the bearing is violated, then a torque is put on the bearing and a bending stress on the pushrod. Obviously this situation should be avoided. Also, there should be no freeplay in the connection between the pushrod and connecting component. Washers should be chosen as well as a bolt clamp up torque to ensure there is no side way movement at the connection between the pushrod and attached component.