The T-seal has an elastomeric bidirectional sealing element resembling an inverted letter T. This sealing element is always paired with two special extrusion-resisting backup rings, one on each side of the T. The basic T-seal configuration is shown in figure 7-2, view A. The backup rings are single turn, bias cut, and usually made of TFE, molybdenum-disulfide-impregnated nylon, or a combination of TFE and nylon. Nylon is widely used for T-seal backup rings because it provides excellent resistance to extrusion and has low friction characteristics.
The special T-ring configuration adds stability to the seal, eliminating spiraling and rolling. T-seals are used in applications where large clearances could occur as a result of the expansion of the thin-walled hydraulic cylinder. The T-ring is installed under radial compression and provides a positive seal at zero or low pressure. Backup rings, one on each side, ride free of T-ring flanges and the rod or cylinder wall (fig. 7-2, view B). These clearances keep seal friction to a minimum at low pressure. When pressure is applied (fig. 7-2, view C), the T-ring acts to provide positive sealing action as fluid pressure increases. One frequently used T-ring, manufactured by Greene, Tweed and Company, (called a G-Tring®1), incorporates a unique, patented backup ring feature. One corner on the ID of each radius-styled backup ring on the G-Tring® set has been rounded to mate with the inside corner of the rubber T. Figure 7-2, views B and C, shows the G-Tring®.
There is no military standard part numbering system by which T-seals can be identified. In general, each manufacturer issues proprietary part numbers to identify seals. However, it is common practice to identify T-seal sizes by the same dash numbers used for equivalent O-ring sizes (discussed later in this chapter) as defined by AS568 and MS28775 dimension standards. Typically, an O-ring groove that accepts a certain O-ring dash number will accept the same dash number T-seal.
In the absence of an existing military standard for identifying T-seals, a new and simple numbering system was created to identify T-seals required for hydraulic actuators (piston seals only) without reference to a particular manufacturer’s part number. The Navy number is composed of the letters G-T followed by a dash number of three digits and one letter, R, S, or T (for example, G-T-217T). The three digits are the appropriate O-ring size dash number according to AS568 or MS28775. The letters R, S, and T designate the number of backup rings that the groove of the T-seal is designed to accommodate: none, one, or two, respectively.
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