Negating the cantilever effect for this latching mechanism

[jeffberube]

Please excuse my poor explanation in advance, I'm not an engineer. I have a latching mechanism that has an off center piston and needs to get actioned from a 'cantilevered' position. Please see attached (crappy) sketch of desired effect vs real world behaviour. Is there any solutions to negate the cantilever effect?

 

[.Scott]

There are a few things that can be done:

Make sure that there is a good fit with the piston and the bushing.
Make sure that the bushing fits snugly into the drilled hole.
Make sure that the material that is holding the bushing is rigid.

After that check list:

You may want to extend the bushing so that the piston comes in contact with only the bushing.
You may want to apply a lubricant to the piston.

Finally:
If none of that works, you may want to put apply two Teflon pads, one to the latch surface and the other to the bulk material surface. This will keep the latch from tilting towards the bulk surface - and provide some lubrication.

 

[jrmichler]

The force of the latch is side loading the Delrin bushing enough that normal friction causes binding. There are multiple things you can do, and you may need to do all of them.
1) Reduce the hook force by adding a lubricant or plastic pad to the latch.
2) Make sure the piston slides freely. This means a smooth surface, and has correct clearance to the Delrin bushing.
3) The Delrin bushing should be (approximately) at least twice as long as the distance from the piston to the latch.

 

[Tom.G]

You don't give any dimensions of weight, but here is an idea for a general solution.

The "hook" (lifting tool) needs to be a tight fit in the "latch" (work) piece so the cantilever force is controlled by the walls in contact with the lifting tool. A 'push button ball lock pin' is one idea that comes to mind; it would replace the existing "hook."

Image from: https://www.pivotpins.com/products/root/1818

The balls at the far end are held extended by a central rod that goes axially thru the pin. When the push button is depressed, a groove in the central rod is moved into position behind the balls so they can retract. This allows insertion and removal of the pin from a hole in the work piece. For more secure locking, a matching groove in the work piece for the balls will do the trick (or just a thru hole in the work piece.)

That's just one off-the-shelf approach. If you are talking really large, build your own tooling. In general anything that snugly grips or inserts in the work piece over a distance will restrain the work piece rotation.

 

[berkeman]

I'm not an engineer. I have a latching mechanism that has an off center piston and needs to get actioned from a 'cantilevered' position.

Welcome to the PF.

I'm not an ME, but that design looks to have problems. Is it something that you bought somewhere and you are trying to make it work? Or something that you or a friend want to design?

To get clean horizontal movement of the female latching piece, the force from the male piece should be in-line (coaxial) with the piston. Otherwise you end up with binding problems like what you are seeing. Can the female latching piece be redesigned to move the piston up closer to the contact point with the male latching piece?

Or, can the piston be supported on a pivot, so it can follow the tilt of the female latching piece? It seems to me that as drawn, this mechanism will not work well, even with the bandaids suggested by others so far. Just my 2 cents...

 

[jeffberube]

Welcome to the PF.

I'm not an ME, but that design looks to have problems. Is it something that you bought somewhere and you are trying to make it work? Or something that you or a friend want to design?

To get clean horizontal movement of the female latching piece, the force from the male piece should be in-line (coaxial) with the piston. Otherwise you end up with binding problems like what you are seeing. Can the female latching piece be redesigned to move the piston up closer to the contact point with the male latching piece?

Or, can the piston be supported on a pivot, so it can follow the tilt of the female latching piece? It seems to me that as drawn, this mechanism will not work well, even with the bandaids suggested by others so far. Just my 2 cents...

Basically I'm going Steve Jobs on my ME and need function to follow form. He strongly advised against this but I don't listen very well. This mechanism is attached to a box and is used to latch the box closed. I want the button to push in straight and sit bang smack in the middle of the 2 halves. Things need to look a certain way and this is the result.

 

[berkeman]

Even Steve Jobs had a health respect for physics and good design...

 

[berkeman]

Okay, there is a design way to make this work. Keep working on it to see if you can figure out what that is. Think "aligning extensions"...

 

[jeffberube]

Even Steve Jobs had a health respect for physics and good design...

Haha I know, I'm not quite Steve Jobs that way. Basically, how do I make this physically possible? Currently the fit of the delrin bushing and the piston is fairly loose as t

Okay, there is a design way to make this work. Keep working on it to see if you can figure out what that is. Think "aligning extensions"...

Any chance you can spill the beans? Because I'm 80k deep into product development here and bank account was dry 20k ago :S

 

[berkeman]

Any chance you can spill the beans? Because I'm 80k deep into product development here and bank account was dry 20k ago :S

Ouch. Extend the piece that includes the male latch piece to have a slider brace that extends down on the left of the female part. Constrain the male latch + extension piece to only be able to slide vertically. Make sense? Can you draw that variation of the design?

 

[JBA]

What is the purpose of the piston and there must be more to the assembly than you are showing, i.e. what holds the sliding component in the outward latched position and what retains the latch from falling off when the top cover is removed? You really need to reveal more of the bottom container around the latch assembly.

Something else not yet discussed is that the tilting of the bottom face latch that will result in it having a sloped contact surface, under load, could result in the top hook forcing the latch inward and releasing the top hook.

 

[jrmichler]

A picture can be worth many words. Here is a free body diagram of the latch shown in the OP. The hook pushes against the latch. There is a normal force Fn, and a friction force associated with the normal force muFn. The resultant of those two forces is the total force against the latch. The reaction forces F1 and F2 are from the Delrin bushing, they act at each end of the bushing. If the resultant force vector is extended, and acts between F1 and F2, then forces F1 and F2 are both vertical upward as sketched. If the resultant force vector acts to the right of F2, then the entire latch tips as shown in the OP and force F1 is downward. When that happens, the assembly binds up.

Possible solutions:
1) Move the hook/latch contact surface down so the resultant force vector acts inside the Delrin bearing.
2) Make the Delrin bearing longer to the right. Extend far enough to get beyond the extended resultant force vector.
3) Make the Delrin bearing longer in either or both directions. The force causing binding is proportional to the distance between F1 and F2. You can get away with the resultant force vector outside the Delrin bearing if the bearing is long enough. Extend the Delrin bearing completely through the part, lengthen the piston, and see what happens.

 

[CWatters]

Not sure how much freedom you have to change things but here are two ideas...May need to add a spring to the first.

PS: The second suggestion assumes the latch is part of an injection moulded lid or similar. It's a bit tricky to get the material to flex just right. Ideally you would make the mould so the flex point is a bit too thin (latch likely to be too flexible) then inject some samples and test. That way you can remove metal from the mould to make it stiffer. If you do it the other way around you have to add metal to the mould and that's a bit more expensive.

 

[jeffberube]

Hi guys, sorry for the late response. Let me show you an outside view of the latch so you can understand your constraints. The original sketch is a cross section of this mechanism. The 'hook' is molded into the upper subframe. The latch molded into the button. This needs to look good closed AND opened.

 

[Tom.G]

Instead of having the latch so far above the piston, can you have the latch inside the piston; that is have the hook intrude into the piston?

 

[256bits]

Hi guys, sorry for the late response. Let me show you an outside view of the latch so you can understand your constraints. The original sketch is a cross section of this mechanism. The 'hook' is molded into the upper subframe. The latch molded into the button. This needs to look good closed AND opened.

View attachment 223389

View attachment 223390

What is your tolerance for the hook-latch mating?
Are the mating parts lid and base covered in a flexible compressive lining so the user can push down to close and latch?
Too much tolerance and the lid is a little bit loose on the base, which may be undesirable for a high end product.
Too little tolerance, or negative tolerance ( which can happen with use with say bent hinges ) and the lid will be difficult to close and latch, or it will not close and latch. Long lasting life of the product would be one consideration, and if the latch is the weak point...
I might want to stay away from sliding pistons, as they also have a tendency to over time stay stuck in a particular position from dirt and grit.

 

[256bits]

I kind of like @CWatters design to the left.

With slight modification

 

[256bits]

The push plate and latch are two separate pieces, that rotate either about the same centre or different centre
Push plate can move latch clock wise to unlatch, and counterclockwise to latch.
Push top of push plate to close if needed and the latch and hook due not connect on their own.
Push bottom of push plate release latch and hook to open.
Spring #2 holds push plate in normal position.
Spring #1 hold latch in close position.

Giving this as another option of many to contemplate.

 

[Tom.G]

If this is a piece of luggage, it might be a good idea to recess the button and/or have an additional interlock on the latch. As it is shown, there is a good chance the button will be activated when being tossed around by baggage handlers.

Cheers,
Tom