Could need some help with a design problem

[et.teljemo]

Hello,

I'm trying to get a mechanism that consists of a shaft you can move back and forth through a hole.
At the same time, the hole must be able to move sideways.
This needs to be super stable without any gaps. You should be able to hold the end of the shaft when it is extended and move it in both X and Y.

In my first and so far version, I have made a house with a milled-out slot where I put in a specially made bushing with flange.
The bushing can then move sideways in the house and is hold in place with the help of sides mounted on the house.
The bushing has a hole where the shaft can move back and forth.
Everything here is done with high precision where, among other things, the bushing and housing are leveled together with a "precision grinder" to provide minimal room between the sides and the ends of the bushing.

This does not work!
It is possible to pull the shaft back and forth in the bushing and I can move the hole sideways if I hold the shaft on both sides of the housing.
If you grab the tip of the shoulder and try to Puch it sideways, it locks directly as the "bureau drawer effect".

I'm thinking about trying out a new version with a bushing where the ends have a much larger surface. More like a hockey puck. However, this is entirely based on guessing.
Another option I thought of is if the bushing would have a flange at both ends to guide it straight easier ...

Do we have anyone here with experience in similar construction?
Is there a good calculation to make this happen?

[g-andresen]

Hi,

try it this way

Screencast

Instead of the contact set, you can also define joint limits.

[et.teljemo]

I have done it like that in fusion 360, with joints, limits and sets. But this doesn't show the actual result when manufactured.

Your design example is just for showing the joint functionality i guess or do you think a short bushing inside the house could help. It feels like the turning force do more damage in that case.

Here is my model btw.. Ill attach it in the main post also..

https://a360.co/2T6XHAE

The model works great but the part manufactured irl doesnt.. So, how do I design it to actually work? 🙂

[g-andresen]

Hi,

If you need an additional rotational movement, use the "Cylindrical" option.

However, you decide whether you need an additional ball bearing for this.
Then you do not need the Cylindrical option.

günther

[et.teljemo]

No,

Dont need any more joints.

I need ideas on how the design itself can work in real life?

[g-andresen]

Hi,

For me, this forum serves as an exchange in the use of Fusion's tools, but not as a guide for constructive questions.

günther

[MichaelT_123]

Hi Mr ET.Teljemo,

"Do we have anyone here with experience in similar construction?"

No, I don't!!!

I looked at the problem from a physics perspective and I sense that the problem you face steams from the balance of static friction forces of two moving elements.  The static friction (highly nonlinear phenomenon) can be described as a force necessary to break the sticking force between components before moving. Many factors are in play here: Van der Waals forces, surfaces roughness, lubrication, etc., which are very difficult to quantify.

My gut feeling is that the stiction force for bushing is much greater than for the rod. Additionally, tangents of vectors of applied forces act as amplifiers of the imbalance.

How to lower the former?

You are the only person who could make a holistic decision here. 

I could only suggest adapting a typical method of building in a 'rolling interface' between the bushing and the housing in the form of strategically placed channels with small ball bearings or rolling pins.  

Regards

MichaelT

[TheCADWhisperer]

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@et.teljemo wrote:

I need ideas on how the design itself can work in real life?

---

1. Where are your sketch dimensions?

2. Is it possible to manufacture perfect parts in the real world or is there a tolerance associated with each and every dimension?

3. Have you allowed appropriate clearances between moving components?

4. Can two soldiers (let's call them atoms) from opposing armies occupy the same space?

[MRWakefield]

There's a few things I'd do to improve the general design as well as reduce the chance of jamming. Obviously I'm not in full knowledge of the entire design intent, restrictions, function or equipment you have available so some of this might not be relevant or practical in your situation. I've not interrogated your model for clearances etc., this is just my opinion based on a quick look.

• At the moment there is only line contact between the bush and the housing which is not ideal (increased wear and possibility of 'notches' being worn into the housing).  Making the bush a rectangular block rather than round will increase the bearing surface area and eliminate these potential problems. You'll obviously need to change the shape of the hole in the housing to suit!
• Minimise the amount that the bush is able to tip by:
  • Maximising the end face length (in direction of travel) of the bush as much as practicable (i.e. make it as wide as you can).
  • Reduce the clearance between the ends of the bush and the end-plates to a minimum.
• Ensure a good finish on the ends of the bush and mating faces of the end plates. Also ensure that the 'lay' is in the direction of travel.
• Radius (or chamfer at a small angle) the four leading and trailing edges of the bush.

Hopefully the quick mock-up below will make sense of the text above. You might need to split the housing to enable you to mill the rectangular channels (unless you have access to a wire eroder). As I said I don't know what equipment you've got access to so certain aspects of the design will be determined by your machinery.

Hope this helps.

 

[et.teljemo]

 

Hi, 

Thanks for the answers 🙂

I have a lot of resources to make really fine parts and I will start out with picking out two of your ideas and apply on the already manufactured part.

At the moment this part is super tight in all directions. About 0,005mm maximum between the ends of the bush sides and end pieces.

Since I believe this is where the parts get stuck Ill try to tune this is with your ideas.

1. I will polish the inside of the side pieces and the ends of the bush to minimize friction.

2. I will add a small chamfer or fillet to the ends of the bush to prevent it from "cutting" in the side pieces.

Ill let you know how it goes!