The two-hand, anti-tie down circuit schematic in Figure 2-12 uses ANSI air logic symbols to simplify schematic drawings. However, most mechanics do not understand the hardware behind the symbols. An electrician may recognize the symbols but often does not understand how air logic functions. So like most problems with hydraulics and pneumatics, changing parts, turning knobs, and swapping lines continues until the machine starts working or an expert is called.





















To make the cylinder in Figure 2-12 extend, depress both palm buttons at the same time and hold them shifted. Tying either palm button more than one second before actuating the second palm button keeps the cylinder from moving. Depressing the second palm button within one second after the first palm button makes the cylinder extend and stay. Letting up on either or both of the palm buttons causes the cylinder to retract. This means that if the operator tries to use one of his hands to adjust or hold a part, the cylinder retracts. To start another cycle, release both palm buttons to reset the time delay. Both of the operator’s hands must stay on the palm buttons when the cylinder is extending.

Notice that the AND1 and OR1 elements on the left receive signals from the palm buttons at the same time. AND1 uses both signals to get an output while ORr1 gives an output when depressing either palm button.

Actuating palm button PB2 sends a signal through OR1 to start TIME ON DEL1. After approximately one half to one second, TIME ON DEL1 opens, sending a signal through normally open NOT1 to close normally open NOT2. Depressing PB1 after NOT2 closes gives an AND1 output, but it cannot go through to shift the directional valve. Actuating either palm button separately blocks the signal to shift the directional valve at PB2.

Shifting both palm buttons concurrently sends a signal through OR1 starting TIME ON DEL1. At the same time, an output from AND1 passes through NOT2, shifting the directional valve to extend the cylinder. The output ofNOT2 also closes NOT1, blocking the output from TIME ON DEL1. Depressing and holding both palm buttons extends the cylinder and keeps it there.

Releasing one palm button while the cylinder is extending drops one output of AND1. When AND1 drops out, the directional valve spring returns, the cylinder retracts, NOT1 opens, and TIME ON DEL1 output closes NOT2. Depressing the released palm button again leaves the cylinder retracted because TIME ON DEL1 closes NOT2. To start another cycle, release both palm buttons to reset TIME ON DEL1. Figure 2-13 shows this operation using ISO valve symbols.