overgenomen uit the Engineer, 1887
The main slide valve has two coupled shells. The inner shell is the actual distributing valve. In the fully left position, it connects the left cylinder end with the channel B and the right cylinder end with the channel A. When the main slide valve moves to the right by the action of the eccentric, it closes both ports. In the fully right position, it connects the left cylinder end with the channel A and the right cylinder end with the channel B.
The outer shell insures that there is always the full (fresh) steam pressure acting on the outside of the main slide valve, so this is kept pressed on the valve face.
Next to the main slide valve, a second slide valve (the direction-control valve) is placed. This can be moved by hand from left to right (and back) to reverse the direction of the steam winch.
In the fully left position, channel A connects to the exhaust and channel B to the fresh steam. So the main slide valve has inside admission.
In the fully right position, channel A is connected to the fresh steam and channel B to the exhaust. So then the main slide valve has outside admission.
There is always the full (fresh) steam pressure acting on the outside of the control slide valve, to keep this pressed on the valve face.