overloads. If the design is not well thought out, however, the stiffness of a 

 joint incorporating a harmonic drive unit might be less than that of a joint 

 using a spur gearing because of the natural flexibility of the flexpline. 



2.2.5 Sealing Techniques 



A significant problem in designing e 

 water use is to choose the right sealing 

 intrusion or loss of oil. Compensating 

 fluid-filled motor housings reduces the 

 chosen for the rotary seal between the i 

 units (see appendix A). The 0-ring is s 

 the gland in the retainer ring by about 

 sectional diameter. This compression ab 

 inner and outer housing and forces the 

 metal surfaces. To seal the potentiomet 

 one 0-ring is used in the so called "cru 

 the 0-ring is crushed into a triangular 

 which has 92.5% of the volume of the 0-r 



lectromechanical systems for under- 

 techniques to prevent seawater 

 the pressure differences by using 

 problem significantly. An 0-ring was 

 nner and outer housing of the motor 

 queezed between the inner housing and 

 10 to 15% of its original cross- 

 sorbs the tolerance stackup between 

 -ring into microscopic grooves on the 

 er cover against the outer housing, 

 sh" installation. In this instance, 

 space between both motor unit parts 

 ing. 



Compatibility between the 0-ring and the fluids with which it will be used 

 is another major design problem. Nitrile or Buna N (NBR) is today the most 

 widely used elastomer in the seal industry. In fact, most military rubber 

 specifications for oil-resistant 0-rings require nitrile-base compounds. 

 Because of their fair chemical resistance and their poor resistance against 

 ozone, which might be generated by brush arc-ing, 0-rings made of fluorocarbon 

 rubber (FPM) were chosen. Table 1 shows the advantages of FPM against NBR. 





Nitrile or 





Factor 



Buna N 



Fluorocarbon 



Ozone resistance 



Poor 



Excellent 



Weather resistance 



Fair 



Excellent 



Chemical resistance 



Fair/good 



Excellent 



Oil resistance 



Excellent 



Excellent 



Abrasion resistance 



Good 



Good 



Set resistance 



Good/excel lent 



Good/excellent 



Acid resistance 



Fair 



Excellent 



Water resistance 



Fair/good 



Fair/good 



Flame resistance 



Poor 



Excel lent 



Dynamic properties 



Good/excel lent 



Good/excellent 



Age resistance 



2 to 5 years 



5 to 10 years 



Table 1. Comparison of nitrile-based versus fl uorocarbon-based seals. 



All other metal -to-metal connections which are exposed to seawater were 

 sealed with anaerobic adhesive, in which single-component, solventless 

 metallic ions are present. These compounds can fill the entire space (from 

 about 0.001 to 0.030 inch) between metal parts and harden without shrinkage. 

 The cured adhesives form a bond that conforms to the adjoining surfaces and 

 prevents leakage, and can withstand corrosive action of most hydraulic fluids 



14 



