ACTIVIDADES POSTERIORES

3.2.1. Hydraulic Supply Pressure

Before the sizes of the hydraulic cylinders could be considered, a value for the hydraulic circuit pressure first had to be chosen. A maximum internal pressure of 10000 psi, <69 N/mm2) is required for the forming process. However, operating at this pressure would make the cost of the hydraulic components very expensive. Alternatively, a lower supply pressure could be chosen and this pressure increased through a pressure intensifier to provide the forming pressure. The second route was taken, with a main circuit pressure of 2500 psi. (17 N/mm2) - sufficiently high as to reduce the size of the hydraulic cylinders to a managable size, but not too high as to make the hydraulic circuit very expensive.

3.2.2. Hydraulic Cylinders applying Axial Force

forming, two hydraulic cylinders would be required, each providing a maximum axial compressive force of 200 kN. The size of the cylinders required to provide this force was calculated as follows:

In order to provide axial deformation of the tube during

Force Required = 200 kN

Supply Pressure 17.2 N/mm2

Area of Piston = EoLCg-Requicad

Supply Pressure = 200 x 103 .mm2

17.2

The actual cylinders used on the machine are two "Mecman" Series 203 cylinders with a diameter of 125 mm and a stroke of 100 mm. This stroke was considered suitable to allow insertion of a 150 mm tube blank into the die with the plungers withdrawn and to provide sufficient axial deformation of it during forming. It also allows the use of longer tube blanks which may be required for more complicated component shapes.

3.2.3. Hydraulic Cylinder applying Clamping Force

The size of the cylinder required to keep the two halves of the die together during forming was calculated in the same way. However, this cylinder was required' to provide a force of 300 kN in order to restrain the internal pressure. Hence :

Force Required v = 300 kN Supply Pressure = 17.2 N/mm2 Area of Piston = Force Required

Supply Pressure

= 30.0 X IQ3 mm2

17.2

Diameter of Piston = 2 x / 300 x IQ3 . 1_ mm

17.2 7i

/

= 149... am

This led to the choice of a "Mecman" Series 206 cylinder with a diameter of 160 mm ( 150 mm was not available ) and a stroke of 150 mm. In this case the stroke determined how far the dies could be opened. The dies blocks were required to open in order to place a tube blank in position, and to remove the formed component after forming. A gap of about 150 mm between the two was determined to be sufficient to allow access. However, in order to provide a clamping force not all of the

stroke is used up, and the actual gap between the two die blocks is about 140 mm.

3.2.4. Hydraulic Pump and Electric Motor

The power source for the hydraulic system is a "Sperry Vickers" variable displacement piston pump - model PVB 5 - driven by a 7.5 kV electric motor. This pump is capable of operating at pressures of up to 21 N/mm2, but is set at an operating pressure of 17.5 N/mm2.

Therefore, the maximum theoretical forces that can be applied by the hydraulic cylinders are as follows ( neglecting any losses ).

For the cylinders applying axial compressive force: Area of Piston ( from manu. spec. ) = 123 x 102 mm2 Pressure

Maximum Force

= 17.5 N/mm2

= 123 x 102 x 17.5 N = 215 kN

For the clamping cylinder:

Area of Piston ( from manu. spec. ) = 201 x 102 mm2

Pressure = 17.5 N/mm2

Maximum Force = 201 x 102 x 17.5 N

= 352..M

The maximum flow rate of the hydraulic pump is 32 1/min according to the manufacturers specifications, but less than this will be obtained when operating at 17.5 N/mm2 and using only a 7.5kW motor. This gives a maximum theoretical flow rate of:

Driving Power = 7.5 kV

Delivery Flow Rate = 7.5 x 103 m3/s = 0.429 x 10-3 m3/s 17.5 x 106

= 25.7 1/min

The actual delivery flow rate will in fact be a little less than this due to the volumetric efficiency of the pump being less than 100%, and due to losses in the circuit between the pump and the cylinders. However, this value can be used to calculate the approximate maximum velocities of the pistons of each cylinder. For the clamping cylinder, assuming all the flow is directed to it, the velocity during the outward stroke < closing the two die halves ) is:

Area of Piston = 201 x 10-4 m2 Delivery Flow Rate = 0.429 x 10-3 m3/^ec Piston Velocity = 0.429 x 1Q~3 m/sec

201 x 10"A = 21 mm/sec

Hence a full stroke of 150 mm will be completed in just over 7 seconds. For the return stroke ( opening the die ) the velocity will be faster, due to the smaller piston area around the piston rod.

Area of Pull Side

of Piston = 137 x 10-A m2

Piston Velocity = 0.429 x 10~3 m/sec 137 x 10~A

= 31 mm/sec

This gives a return stroke time of under 5 seconds.

For the cylinders applying the axial compressive force a similar approach is used again assuming all the flow is directed to them C in practice, however, some of the flow will be directed to the internal pressure ). For the outward stroke ( causing axial deformation ) the velocity is:

Area of Piston = 123 x 10~* m2

Delivery Flow Rate = 0.429 x 1Q~3 m3/sec

to One Cylinder 2

Piston Velocity = 0.429 x 10~3 m/sec 2 x 123 x 10~* = 17 mm/sec And for the return stroke:

Area of Pull Side = 84.2 x 10“* m2 of Piston

Piston Velocity = 0.429 x 10~3 m/sec 2 x 84.2 x 10-*

= 25 mm/sec

This gives an outward stroke time of under 6 seconds and a return stroke time of 4 seconds to complete the 100 mm stroke of both cylinders.