I'm making a game where you have to hit targets, kind of like whack-a-mole except the targets aren't actively actuated. I'd still like an element of mechanical "feel" to it, so I'm aiming to put the targets on a rod with a spring, so that when you whack it, it goes down, triggers a limit switch so the game knows you've hit it, and it springs back up. See below for a rough model. The intention is to use an off-the-shelf shaft, spring and shaft collar.

A simple spring mechanism

In terms of scale, the target is going to be somewhere in the range of cue ball to grapefruit, and made from a light material (e.g. plastic, resin, etc), so the spring doesn't have to be particularly strong. (I realize this is slightly vague, but it's TBD and I'm only tasked with making the mechanism.)

It needs to withstand a few hundred whacks per day for a couple of months. The mallet will be coated in thick foam but wielded by members of the public.

Would using linear bearings in the brackets or a linear bearing pillow block as the whole bracket be overkill?

Is there an off-the-shelf component I could use for the bracket otherwise, or would I be better to have a metal bracket fabricated? I've hunted around for a while, but I don't know what to call that kind of bracket.

I've also considered 3D-printing a bracket, but I'm not sure if it will be strong enough to put up with repeated whacks. Might I get away with it if I designed enough bracing into the bracket?

The limit of the travel can be set by a fixed surface (e.g. the cabinet) so theoretically the bracket primarily needs to withstand the force applied to it by the spring under compression, but if the target is whacked diagonally, the bracket must withstand the lateral force too.

Another option I have considered is laser cutting 1/2" ply to make a box-jointed housing, but not sure if the wood against the metal will make a kind of jittery movement. I'm imagining kind of like below, but with box joints and probably an extra sort of lintel under the horizontal pieces.

A different housing

I have time and resources to try a couple of simple prototypes, but I can't feasibly whack it thousands of times to see if it gives.

  • $\begingroup$ Big question: how many of these do you need? There's a huge difference between making 10 and making half a million. $\endgroup$ Commented Sep 22, 2023 at 16:00
  • 1
    $\begingroup$ It'll be somewhere in the region of 20 $\endgroup$
    – sjmeverett
    Commented Sep 22, 2023 at 21:41

1 Answer 1


I'd probably make the brackets out of a low-friction plastic like nylon. You could 3D print them, but they are (or at least can be) simple enough that there may not be much point.

Assuming you're only making a relatively small number, I'd probably just get rectangular nylon bar stock, drill a suitable-size hole and cut it to length (and probably a couple screw holes to mount it). No need for the L-shaped mounting base--just use thick enough material to mount as rectangle. Something along this general order:

enter image description here

That'll use a little more material than the L-shaped bracket you've drawn, but the savings in fabrication time will more than compensate. Given the application, it doesn't seem as if the slight increase in friction from a larger bearing surface is at all likely to matter.

At the opposite extreme, if you were going to be making millions of these, it would probably make sense to get molds made, and have essentially the entire mount injection molded. This has much higher initial cost (molds are pretty expensive) but reduced unit cost.

As far as dealing with lateral forces, I'd mostly try to design it to minimize the ability to exert lateral force in the first place. If you look at a typical whack-a-mole kind of game, the button that sticks up won't be a hemisphere. It'll have a much larger radius, so the user has no way to hit it that produces significant lateral force. The "button" that sticks up is also a tight enough fit in the hole that you can't really move it very far laterally in any case.

You could also use a shaft that's flexible enough (e.g., probably also plastic) that most lateral force would simply flex the shaft rather than being transmitted to the brackets.

  • $\begingroup$ Thanks, I'm probably over thinking it! $\endgroup$
    – sjmeverett
    Commented Sep 22, 2023 at 21:41

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