As I started working on the Turtle robotic arm design I was looking for any articles and tutorials how to efficiently build the arm joints. I needed them to be as simple as possible yet sturdy enough to withstand any unexpected loads that can occur for ex. when the user drives the rover into a wall. All I could find was an academic literature covering only industrial uses that are designed for a way too long life-expectancy at the cost of the bearing weight and complexity.
The hardest part of robotic arm design generally is not to allow for any play in any of the joints – to provide stiff and exact movement of the arm and its sensors. Is it even possible to manufacture such joints without any specialized machinery? I think so!
Today’s topic: how to efficiently connect a gripper to the robotic arm?
I won’t start with the theory but I’ll mix a little bit of it into the text later. I consider practical example as the best lesson – so let’s start with a real task. We have to somehow connect two parts from figure 1 into a joint. First problem we have to deal with is servo inside of the gripper part preventing from using one long shaft – it forces us using two smaller shafts in one axis.
As I prefer practical examples over theory, let’s start with a real task I needed to cope with during the arm design phase. The theory will come later.
The task is to combine the two parts from figure 1 with a rotational joint. As the design needs a servo-motor to be fit in the axis of the shaft we need to use two smaller axis instead of a single long.
Figure 1. Base part of the gripper and ending of the robotic arm
Few extra words: issues with metal bending
Here I would like to stop for the moment and give you a little hint about preparing holes for the shafts in elements bent from steel or aluminum. Bending elements is not really precise, so in 99% of cases holes cut by laser before bending won’t end up in one axis. To make it work you should laser-cut only a one slightly-smaller hole in one side of the element. What about second hole? Put the element on a flat surface and use a bench drill to produce both holes with one pass – use small laser-cut hole as a pilot. As you want to keep tight-fit tolerances of the holes you should re-drill with a reamer at the end. Hole that need to be then reamed should have 0,5-1mm smaller diameter than the nominal size of the reamer – in other case you can damage the tool! Standard fit of any of-the-shelf reamer is H7 – it will be enough for our application.
Selection of the shaft
Going back to the topic: We decided to (or rather we had to) use two separate shafts in one axis. The next step is to figure out how to design/choose our shafts and prevent them from moving in axial direction. First idea in most of our minds – use a bolt, washer and a nut. NO! NEVER! It is very important that the thread of the bolt can never be used as a friction surface. But there is still a hope for all of the bolt fans – fitted bolts! (fig. 2). They have a cylindrical part where the touches the bearing and still have the thread at the end to properly fix it.
Using them as an axis is really easy and economic. The only thing you need to remember when using them is that the bolts length is normalized. It forces your design to fit the bolts– not the other way. The other issue with the fitted bolts is also that the thread has always the same length. It is the same for the bolt of 10mm and 100mm
Unfortunately we can’t use fitted bolts in our case. Why? Look at the figure 3 and the dimensions of the bolt from figure 4. The bolt head is 4,5 millimeter long and we have only 3,25mm of free space. It just won’t fit!
What now? The other way is to use clevis pins as shown in figure 5. These are very cheap and have narrow head.The head is only 1,5mm wide for a pin of 6mm dia.
Now the pin head blocks one degree of freedom, but the shaft could still move axially in one direction. There are a few ways to solve that. Clevis pins are commonly used with small holes and cotters (protecting pins from falling out from holes/bearings) but those require extra machining and a good fit.
I personally love to use circlips at ends of shafts. Circlips essentially are small springy rings that are easy to use and super cheap. There are few types of them – most commonly used are Seger circlips and push-on circlips (figure 6). Seger circlips can be both of internal type (for holes) or external (for shafts). Internal circlips are used for example for keeping ball bearings in their housings. Push-on are only available to be used on shafts.
What’s the difference between them? Seger circlip unfortunately requires groove on the shaft which requires machining. This means – when you are already machining the shaft making an additional groove is not a problem and shouldn’t change the cost of the shaft significantly. On the other hand, when you use any of of-the-shelf products (like here) I suggest using push-on circlips to cut on additional machining cost. Difference is shown on figure 7.
Figure 7. Shaft before and after mounting Seger (on the right) and push-on (on the left) circlip
This article is a starting point of a series about Turtle robotic arm mechanical design.
Now we know how to design an axis and what are the ways to prevent it from unwanted movement, but we still don’t have any bearing to build the joint properly.
I’ll cover the bearing in the next article of the series, hope you like it! If you have any questions, don’t hesitate to ask. See you next time!