represents the force applied to the stairs 10 it can be seen that through the arc of 60 u00b0 to u2212 60 u00b0 the torque supplied in excess of the torque required is approximately 5 kgm when the stairs 10 is in the fully retracted position the force required to overcome the hydraulic force is substantially more in general the retraction mechanism 25 is set up so that with a constant output from a hydraulic ram 40 the torsion forces applied to the stairs 10 exceed the forces required to lift the stairs 10 for approximately u2154 of the arc in the last third of the arc approaching the fully retracted position the induced torque force increases so that the stairs 10 are firmly held in the retracted position and doesn 39 t move excessively while the equipment is in operation examples of the retraction mechanism 25 are given in table 1 the control for the hydraulic ram 40 of the retraction mechanism 25 may be operated through the pilot circuit for the park brakes on rubber tyred equipment or the pilot circuit for the controls on tracked equipment in large earth moving equipment the park brakes are normally on and hydraulic pressure is used to release the brakes alternately the hydraulic pressure for the retraction mechanism 25 may be provided by the main hydraulic system or from a separate hydraulic pump the graph in fig4 depicts excess lift torque for a vehicle ladder or stairs retracting through 90 u00b0 e g from near vertical when deployed to near horizontal when fully retracted the graph plots lift torque kgm over and.