The secondary gearbox serves as a mechanism for increasing torque, shifting the torque axis and  splitting the torque output into two axes. The torque tube travels inputs to a gear train that shifts the  torque to a planetary train similar to that used in the PDU. Because the locking and folding shafts must  operate at mutually exclusive times a timing mec hanism, described later in this paper, was designed to  control the motion of the two outputs. The se condary gearbox is pictured in Figures 9 and 10. 

Due to severe packaging limitations, most of the gearing in the secondary gearbox was herringbone  helical gearing to allow maximum torque capability with the smallest possible face width. The gears were  coated with Vitrolube, a ceramic solid film lubricant  capable of withstanding the higher contact loads. The  gear teeth were further coated with a light film of  Braycote 602EF. These gears also were mounted in  aluminum-bronze bushings with fixed center distanc es because there was simply no room for bearings  that could withstand the magnitude of  load required to react the tooth loading. The housings were hardanodized with the bushing holes being ma sked and chemical conversion coated. 

Secondary Gearbox Testing   Once fully assembled, the secondary gearbox underw ent benchtop testing in order to measure the  efficiency of the gearbox, necessary for torque margin determination and proper rigging of the  mechanisms. The efficiency, as expected due to t he necessity of using bushings under high loads, was  poor. The gearbox also underwent thermal-vacuum te sting with the torque limiter and PDU, and went  through fin random vibration testing. In all case s, the secondary gearbox operated with no problems. 

Secondary Gearbox Lessons Learned   • Use hex-head fasteners whenever possible.  Originally many of the fa steners, of which there was a  fairly large number for sealing purposes, used o ffset cruciform recess heads which can be very  difficult to install to proper torque values and even mo re difficult to remove. Every fastener that could  be swapped was replaced with a hex-head version and assembly became much easier.  • If you can’ t use bearings, you’ ll pay the price in efficiency. 

Timing Mechanism Description   The original design concept for the timing mechanism was very simple and consisted of two discs similar  to Geneva mechanisms mounted on the two output shafts (See Figure 8). A simple test article was  developed to explore the concept. It became immediat ely apparent that though simple, the high torques  involved (282.4 N•m (2500 in•lbf) in one shaft and 50.8 N•m (450 in•lbf) in the other) produced very high  sliding friction forces on the discs which used all of  the torque from the gearbox and allowed no motion. 

Latch Disc  Fold  Mechanism  Disc  Figure 8.  Genev a Mechanism Timing Discs   A second concept was developed to reduce the fricti on in the system. This was accomplished using a  cam/roller/linkage concept that emphasized rolling fric tion rather than sliding friction. While the new  design reduced the friction in the system, it also greatly increased the complexity and size of the  mechanism. This larger design was moved from t he output shafts to an open space between the gearbox  housings (Fig. 9).  372