The Screw Jack

An interesting record of the project development of an identical mechanical challenge – ‘locking the position‘ of a heavy vertical telescopic pole, at

Attempt #5 – Final Locking Prototype

Now that the locking mechanism was functioning, I still had one problem left. The spring action to ensure that the locking tooth has a constant forward pressure was not going to last very long as I could see it already showing signs of stress from use.

The first attempt, as you can see in some of the previous photos, uses an American football shaped lobe that rotates between two pieces of hard maple. When rotated backwards ,the football shaped lobe spreads the maple fork creating pressure. When released, the maple fork closes down on the lob returning it to the locked position.

Locking spring attempt #1 shot 1 Locking spring attempt #1 shot 2

Even though this attempt at an all-wood spring was clever, it was bound to break, or at least tire over time. Instead of showing a visible spring, I thought it would be more elegant to hid it inside of the locking tooth support arms. By hooking a small spring round two dowels inside of a cavity hidden by the nut and washer, the tooth was then reliably forced into the locking position each time. Should the spring break or tire, it can easily be replaced.

Locking spring attempt #2 shot 1 Locking spring attempt #2 shot 2 Locking spring attempt #2 shot 3

Here is a video demonstration of the final prototype. Pardon the messy shop =)


Here are a few more images of similar structures


Leave a Reply