How the Master 175 Padlock Works

On the left is a picture of an assembled lock after the bottom two thirds of the body shell was cut off. On the right, the padlock has been dissassembled, showing some of its parts.
The thumbwheel lock has some similarities to the single dial lock. Both locks have cams. The single dial lock has 3 and the thumbwheel lock has 4 cams. Both locks have a pawl. The left hand picture shows the pawl in its normal position within the lock. In the right hand picture, the pawl has been removed and is to the left of the lock body. The pawl is somewhat fork shaped and has four tongs, facing down in the picture. At the top of the pawl, there is a "yoke" which fits into the deep notch in the lock body, at the top of the lock body. The pawl pivots on two notches, on the left and right sides of the hole or cavity in the lock body. A spring holds the pawl in place. In the left hand picture, the end of the spring is a dark circle in the middle of the pawl. In the right hand picture, the spring is partly visible near the second tong from the left.

The latch has two pieces, in the upper right of the right hand picture. There is a spring between the two pieces to push them apart. In the normal locked position, the two pieces of the latch fit into the notches in the shackle. The shackle is pushed in the unlocked direction by a spring (not visible.) Because of the beveled surfaces in the shackle and on the latch pieces, the two latch pieces are pushed together as the latch spring pushes the shackle toward the unlocked position. The latch pieces squeeze the yoke and hold the pawl so it won't move. The yoke blocks the latch pieces from moving together far enough to come out of the notches in the shackle, keeping the shackle in the locked position. With the yoke squeezed and held in place, the lock will not open even if the correct combination has been dialed.

When the shackle is pushed down, the spring between the latches pushes the latches deeper into the shackle notches. The yoke is no longer "grabbed" and is free to move. The pawl's spring pulls the tongs down against the cams. If all four flat edges of the cams are facing the tongs, the tongs move down far enough to raise the yoke out of the way of the two latch pieces. If the shackle is released, the latch pieces will move together far enough to allow the shackle to be released and the lock will open.

With the lock open, the yoke is resting on the left latch piece. If the dials are turned, so they no longer show the correct combination, the fork is pushed up and "floats," no longer resting on the pivot points. The pawl's spring and the cavity walls keep the pawl from moving too far. When the shackle is pushed back into the lock, the latches fall into the shackle notches far enough to allow the yoke to drop between them. The yoke keeps the latches far enough apart that the shackle can not be released.

If the dials had not been turned while the lock was open, the dials would still show the correct combination and the pawl tongs would be resting on the flat edges of the cams. The yoke would still be above the gap between the two latches. If the shackle were released, it would pop out of the lock. Thus, the user must move the thumbwheel(s) to a wrong combination in order to get the lock to close.

So, once it is locked, the lock will not just pop open when the correct combination is dialed. After dialing the correct combination, the user pushes in the shackle and releases it to get the lock to open. This feature makes it more difficult to get the combination by trying all possible combinations. The shackle must be pushed in and released after each combination is dialed.

Once the lock is open, it can not be locked until the thumbwheels show a wrong combination. Therefore, the user can not close the lock and accidentally leave it showing the correct combination. On the other hand, a careless user can leave the lock showing a partial combination.

Changing the Combination

The combination change tool pushes the cams to the right, disengaging them from the thumbwheels. The thumbwheels can then be set to a new combination. When the change tool is removed, the thumbwheels re-engage with the cams.

The lock mechanism will not allow the combination to be changed unless the thumbwheels show the current combination. This feature ensures that, when the change tool is removed, the thumbwheels will show the correct new combination. The way this works is as follows:

The cams have narrow notches opposite the sides with flat notches. Unless the narrow notches are lined up with barriers in the lock body, the cams are blocked from being pushed away and disengaged from the thumbwheels. Thus, the cams can not be disengaged from the thumbwheels when the wrong combination is showing.

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