Hydraulic actuators are commonly used in mechanical systems, and actuator efficiency is one of the most important factors in these systems . The energy loss to overcome friction force makes the actuator less efficient. Wearable rehabilitation robotics is one of the applications of hydraulic actuators. Hydraulic cylinders deliver the power extracted from the external resources and/or less stroke-affected limbs to the more stroke-affected limbs (Fig. 1).
O-ring seal, rolling diaphragm, and gap seal cylinders are three common technologies that have been used in different hydraulic systems for years. O-ring seal actuators use an O-ring seal between the piston and cylinder. Rolling diaphragm actuators have a diaphragm between the cylinder and piston which rolls back and forth. In gap seal cylinders, there is a gap between the piston and cylinder. Since it is a tradeoff between leakage and friction, leakage between the two chambers in these cylinders is tolerated to reduce friction (Fig. 2). One study examined low friction cylinders in a low pressure hydraulic transmission . In this study, rolling diaphragm cylinders were used in the transmission, but the restriction on stroke length of these cylinders is a problem that needs to be solved. Commercial rolling diaphragms are manufactured using compression molding of a sheet rubber and woven fabric , a manufacturing method that limits the stroke length to no more than the bore of the cylinder. Rolling diaphragm cylinders with the higher stroke-to-bore ratios could multiply the work per cycle of the system . Furthermore, there are limitations of using short stroke length rolling diaphragm cylinders  .
A more thorough friction evaluation of various cylinder technologies is needed to determine which technology has the lowest friction and is most appropriate for low pressure hydraulic systems like rehabilitation robots. Developing a low friction, leakage-free cylinder without stroke limitations is needed for small hydraulics.
Using an experimental test, we measured the resistance forces in three types of cylinders: O-ring, gap seal, and rolling diaphragm. The cylinders were tested at low-pressure and with mineral oil to determine the lowest friction cylinder technology. The same friction test was performed in a novel, long-stroke, rolling diaphragm cylinder (LSRD) to compare it in two different thicknesses with commercial actuators.