Internally Anchored Bolt to Secure Steel Robotic Platform

[Brad-y]

Good afternoon! MY name is Bradley I am working at a robotics lab this summer and I wanted to ask for some help regarding a problem I have. The issue is that I need to find a way to attach a platform to a metal block which s the arm of the platform using bolts that will exit from the block and pass through the platform before being secured by a nut. I've seen online potentially using different variation of screws with anchors, however I have trouble deciding what is necessary. For instance i also seen that I could thread holes into the arm and get a double sided bolt to secure the load, however the platform will be swinging and vibrating, which I am concerned may lead to the threads loosening and failing at high loads/frequencies. Any ideas would be greatly appreciated, as well as if anyone could give recommendation based on their experience regarding the strength of the bolts I will need and the method to attach them. I will also note that previous platforms were attached using welds however the professor wants to use bolts instead, which is why if anyone can illuminate whether the anchors are necessary or rather with specific nuts/bolts the platform can be kept modifiable.

An image of the platform and arm structure is below. In the current design the platform and arm are one solid block, but I want to model a realistic attachment between the platform (flat area) and the arm (vertical rectangular extensions on either side).

The SOLIDWORKS file:
[URL unfurl="true"]https://res.cloudinary.com/engineering-com/raw/upload/v1721344723/tips/Platform_new_qz4xph.SLDPRT[/url]

 

[SWComposites]

I don’t understand where (or why) you want to introduce a joint to the part shown in your picture. Please provide some additional sketches of the design concept and assembly.

 

[Brad-y]

Essentially the part shown is a mockup of the final assembly, however it is simplified and shows the platform being made of a single block of steel, when in reality it will be three blocks: 1 platform in the center and two arms to support and translate the rotation from the motor arm (gray rod attached to the arms). I want to make this attachment without welds, as previously done, and want to opt for a nut and bolt assembly but am not sure the best way to attach the bolt into the block so it can come out and penetrate through the platform which will be held with nuts. My question is that I'm not sure if this will be strong enough or if it will loosen from the motion and wanted some input from others who may have dealt with this before?

2D drawing:

My proposed plan is to place the bolts as shown in red:

- The bolts will be internally seated within the block either threaded in, or using an anchor but I need help deciding which is better and what is used normally and what I should be looking for when choosing the bolts/nut specifics.

Showing motion of assembly:
[URL unfurl="true"]https://res.cloudinary.com/engineering-com/video/upload/v1721353819/tips/10mmthickness_with_steel_zbhizc.mp4[/url]

Thank you for any help you can provide I greatly appreciate it!

 

[SWComposites]

That is a horrible configuration for a bolted joint. Sketch shows bolts into tapped holes. These are just a pain and hard to keep tight. And all of the moment in the corners is going to go thru the bolts.

Why does it need to be three separate pieces (other than “prof said so”)? Why not just machine from a single metal block?

When given an engineering problem - FIRST define all of the requirements. Only then look for solutions.

 

[Brad-y]

When I presented the design he mentioned adding attachments because my current model showed it as a single block that was machined, however he said that it would be too expensive and unnecessary and to instead use bolts. Is there a better configuration that you're aware of or what would you do in this situation. I agree that bolts seem like it would loosen and generally not be secured, but I know there are certainly machinery using bolts for even harsher loads and was wondering what methods or types of bolts they use?

 

[SWComposites]

If you must use the current config in 3 pieces, then match drill undersize (thread minor diameter) the parts, tap the holes in the end blocks, ream out the holes in the base to bolt diameter, then assemble with socket head cap screws or hex head bolts. Use high strength steel bolts (not cheap crap ones from local big box home improvement store). Bolt length to be less than tapped hole depth so they don’t bottom out. Torque to high level. And hope they don’t loosen with use.

Bolted joints really should be designed so the bolts are in shear, with room for separate nuts. Yes, tapped holes can work, see engine head bolts, but they can be a PITA to get right.

 

[Brad-y]

Ok thank you for the advice. I plan to use SAE Grade 8 or ASTM Grade BD bolts, but is there any reason you recommend the socket head cap screws of hex head bolts over using a rod and nuts at the end. Is the benefit simply that by having one less threaded connection (cap instead of a locknut) it will loosen less/have less failure points? Also I had a concern when threading the hole after reaming, who can I ensure that the threads between the arm and the platform will match? Or will it involve the arm being threaded and the platform being flush - wouldn't this increase the gap between the bolt and the hole leading to a looser fit?

 

[SWComposites]

Threads go into the arms.
The platform is not threaded. So when the bolts are torqued there is clamp up between the two pieces.
Don’t use threaded rod; how are you going to thread it into the arm? (no, it is not acceptable to have loose sloppy threads). Just use bolts that can be torqued by the heads.

 

[desertfox]

Hi Brad-y

I would suggest you cast the whole thing and make an allowance for machining the critical areas and forget the bolts.

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein