Seeking long servo horns or alternative for motorized, expanding top hat

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1

I am trying to design a magician’s top hat, which can expand from a collapsed state, using remote control. Attached is an image, showing the general concept. I will mention that collapsing top hats are known a “Opera hats” because they were popularized when top hats were common formal wear, and users could collapse them to store under their seat.

These types of hat have a rigid brim, and top disc. Then, there is fabric that connect the two. The traditional design has 4 spring-loaded linkages around the perimeter. This is what I want to motorize.

Expanding top hat
Expanding top hat882×796 7.94 KB

In my picture, the linkages on the servo is 2 3/8" long (center to center). But, I can’t find one this long. Also, I am a little concerned that such a long lever arm is side loading the servo in a way it is not designed to be used. The robot joint systems handle this by attaching a |_| shaped bracket that provides a pivot point on the back of the servo. These won’t work for me, because the way they are designed takes up far too much space. And I need the pivot point to be as close to the permimeter of the hat as possible.

Also, I would love to get a connection to the servo that is not a spline. I am surprised the robotics world has not generated servos that have output shafts with flats on them, or that have shafts going through the entire servo so they can handle a cantilevered load when used with wheels.

I know I can purchase small gear motors with encoders, that have the mechanical features I would like. But, then it gets more complicated to design the control system and the control board starts to get larger, etc. I considered using dumb gear motors and send them to their extremes of travel, where they stall. But, because I need enough speed to open the hat fast, they need to be fairly powerful, and I am concerned they would damage things.

So, I am hoping someone can suggest one of the following,

  1. A servo that has a mechanical connection that is not a spline.
  2. A source for servo horns (that use the typical spline to connect) with at least 2 3/8" between the holes.

Thank you for any replies.

1 Like
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Hi @cadcoke5 ,

I haven’t seen such long servo horns.

If these are very specific to your project, maybe you can consider 3D printing it (if they are from plastic) or machining it from metal?

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I think you will need to customize your servo horns.

1 Like
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Thank you both for your replies.
It looks like my approach will be to use a pre-made servo horn (or disc) and then screw that to a metal rod the length I need.

With RC Servos being so common in the hobby robotics world, I am surprised that I didn’t find any that used a more standard means of connecting to another object. (i.e. a shaft with a flat).

-Joe

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Hi @cadcoke5 ,

If servo motor would use just a flat shaft, it would be harder to attach something to it right?

Spline on the shaft make it more stable and it can withstand more torque without slipping.

6

Most gear motors have a round shaft, with a portion of it flattened. This way you can use a connector that has a set screw on the side that will lock it into place Often called a “D” shaft. To make an attachment, you just need to drill a hole the right size, plus drill and tap into the side of whatever it is. It is something you can create yourself. But a spline has to be done by a manufacturer of the servo horns.