Hydraulic safety system for grinding machines with magnetic table.

Prodan, Dan ; Bucuresteanu, Anca ; Balan, Emilia 等

Abstract: The paper presents the research performed during the modernization of a cup-wheel grinding machine with magnetic table (Botez, 1977) (Diaconescu, 1962). The initial design of the above mentioned machine had the following disadvantage: in case of an accidental interruption of the magnetic table power supply, the workpieces were flung by the grinding wheel, causing the damage of the workpieces and being a serious problem of labor safety. The recently developed solution allows almost instantaneous elevating of the grinding wheel, so that the workpieces are no longer driven when the magnetic table power supply is interrupted.

Key words: hydraulic, grinding machines, magnetic table.

1. INTRODUCTION

As shown in Figure 1, the hydraulic installation is based on a hydro-pneumatic accumulator (Prodan, 2004). The pump P driven by the electric motor ME sucks up the oil from the tank T through the absorption filter F1. When actuating the directional control valve 1D1 powered by the electromagnet E1, the pressure valve Sp ensures the maximum adjusted pressure. This is a line with pre-control, and two pressure switches--1Rp for minimum pressure and 2Rp for maximum pressure--control it. The check valve 1Ss separates the line that includes the accumulator Ac from the pre-controlled line (Bucuresteanu, 2001). The manually actuated directional control valve 1D2 allows the coupling and uncoupling of the safety system. When powering the magnetic table the electromagnet E2 of the directional control valve 1D2 is also powered. In this case the linear hydraulic motors C1 and C2 are uncoupled from the line that includes the accumulator; the disk springs keep the grinding wheel in working position.

In the meantime, through the pre-control line, a pressure within [p.sub.min] and [p.sub.max] is kept in the line that includes the accumulator. If the magnetic table power supply fails, the electromagnet E2 is no longer actuated and the accumulator line discharges towards the two linear hydraulic motors C1 and C2. The motors work against the pre-load disk springs and lift up the grinding head from the working area so that the workpieces are no longer driven away from the table.

2. MATHEMATICAL MODELS FOR THE CALCULATION OF THE SAFETY SYSTEM

Considering that the accumulator has a volume [V.sub.0] and it is filled with nitrogen at the pressure [p.sub.0] and supposing the transformations are adiabatic, then the mathematical model below can be applied (Prodan, 2006):

[p.sub.0] x [V.sub.0] = p.sub.i] x [V.sub.i] = p.sub.(x)] x [V.sub.(x)] = [p.sub.f] x [V.sub.f] (1)

[p.sub.(x) = [p.sub.0] x [V.sub.0]/[V.sub.i] + 2 x S x x (2)

[FIGURE 1 OMITTED]

In the relations (1) and (2): [p.sub.i], [V.sub.i]--initial pressure and volume of nitrogen at the time of starting emptying the accumulator; [p.sb.(x)], [V.sub.(x)]--pressure and volume at the time the wheel is lifted up with instant dimension x; [p.sub.f], [V.sub.f]--pressure and volume at the end of wheel elevating process; S--useful surface of the hydraulic motors C1 and C2.

Considering the mass M of the movable assembly, the disk springs constant K and the coefficient of the force lost in proportion with the velocity b, then the following condition of dynamic balance can be written:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (3)

From operating point of view, the evolution of the speed and of the movement of the working head is important. Specialized programs of simulation are recommended for establishing these values.

3. SIMULATION OF THE SAFETY SYSTEM

For simulation it has been considered the using of an accumulator of [V.sub.0] = 2.5 l, filled up at [p.sub.0] = 80 bar. The working head with the clamping devices and the grinding wheel weigh 2000 kg. According to the catalogues of the manufacturers of hydraulic components, the switching time of the directional control valve is 10--25 ms, when DC driving.

The simulation diagram in Figure 2 has been drawn up based on the above mentioned mathematical model. The speed of the grinding wheel develops in time as shown in Figure 3. The maximum velocity is approximately 0.18 m/s.

The vertical movement x of the working head behaves as shown in Figure 4.

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

It is noticed that after 0.08 s the working head rises by 7 mm, which is sufficient to protect the workpieces on the magnetic table. The time of switching the directional control valve should be added to this value. Therefore, it can be considered that 0.1 s time is enough for a lift up higher than 5 mm. The pressure in the accumulator decreases due to the charging of the line containing the hydraulic motors. This decrease can be noticed on the characteristic shown in Figure 5; pf is approximately 86 bar, if pressure pi is 90 bar.

[FIGURE 4 OMITTED]

[FIGURE 5 OMITTED]

The intervention of the accumulator for lifting up the working head is not a phase of work, it occurs in case of emergency (power failure) only.

If the uncoupling of this system is needed, than the directional control valve 1D2 should be manually actuated. Thus, the accumulator is uncoupled and the line with the two linear hydraulic motors discharges.

4. CONCLUSION

The installation designed and made has been used for a refabricated grinding machine (Prodan & Marinescu, 2005).

When the power supply failed the wheel rose almost instantaneously and the workpieces on the table were not flung anymore.

The results of simulation were confirmed during the testing of the machine.

Simulation is useful when designing. Dynamic calculation allows avoiding some modifications and corrections during testing.

The values of the coefficients such as: K--disk springs constant and b--coefficient of the force lost in proportion with the speed, etc. should be known in order to have a simulation close to reality.

5. REFERENCES

Botez, E. (1977). Masini-Unelte. Bazele teoretice ale proiectarii, (Machine-tools. Theoretical fundamentals in designing) Technical Publishing House, ISBN CZ 621.9, Bucharest.

Bucuresteanu, A. (2001). Acumulatoare pneumohidraulice, ((Pneumohydraulic accumulators)) Printech Publishing House, ISBN 973-652-292-X, Bucharest

Diaconescu, I. (1962). Masini-Unelte (Machine-tools), Technical Publishing House, ISBN CZ 621.9, Bucharest Prodan, D. (2004). Hidraulica masinilor-unelte (machine-tools hydraulics), Printech Publishing House, ISBN 973-718109-3, Bucharest

Prodan, D. & Marinescu, S. (2005). Refabricarea masinilorunelte. Sisteme hidraulice (Re-designing of machine-tool. Hidraulic systems), Technical Publishing House, ISBN 973-31-2255-6, Bucharest

Prodan, D. (2006). Masini-Unelte. Modelarea si simularea elementelor si sistemelor hidrostatice (Machine-tools. Modelling and simulation for hydrostatical elements and systems), Printech Publishing House, ISBN 973-718-572-2, Bucharest