Multi-stage hydraulic stretcher for elongated fasteners

An hydraulic device for an elongated fastener which is to be pre-stressed, said device comprising:

• a. an upper stage comprising:
• an upper portion contacting a traction means mounted on one end of said fastener;
• an upper piston defining a central bore through which a portion of the fastener passes and comprising a first part forming an upper guide skirt and a second part of larger diameter forming an upper head, and
• an upper chamber provided with a shoulder encircling said upper guide skirt;
• b. at least one intermediate stage comprising an intermediate piston defining a central bore through which a portion of the fastener passes and comprising a first part forming an intermediate guide skirt and a second part of larger diameter forming an intermediate head contacting the upper guide skirt of said upper piston, the thickness of said intermediate guide skirt being less than the thickness of said upper guide skirt;
• and
• an intermediate chamber provided with the shoulder encircling said intermediate guide skirt;
• c. a lower stage comprising a lower piston defining a central bore through which a portion of the fastener passes, and comprising a first part forming a lower guide skirt and a second part of larger diameter forming a lower head, and
• a lower chamber provided with a shoulder encircling said lower guide skirt;
• and
• d. means for introducing an hydraulic fluid under pressure into said chambers near said shoulders;
• whereby the deformations due to the compressive forces of each piston on the next upper piston and to the radial pressure from the hydraulic fluid of the guide skirts of the various pistons are substantially equalized.