Mechanism that shrink when pulled - Page 1
Forum section: electronics / general

author: temperance
author_rank: Super Contributor
timestamp: May 18, 2025, 01:26:00 am
content: After seeing Dave's video with him visiting Australian manufacturing week I think it doesn't hurt to post some stuff concerning mechanicsSkipping drill chuck theories, milling machines and lathes and how to work with them without killing yourself it is more interesting to dive into more interesting mechanic. But compliant mechanism aren't that far away from electronics.

author: coppercone2
author_rank: Super Contributor
timestamp: May 18, 2025, 02:48:22 am
content: I seriously don't like these mechanisms very much because something always cracks and it reminds me of snap hinges and stuff

author: CatalinaWOW
author_rank: Super Contributor
timestamp: May 18, 2025, 03:28:43 am
content: Most designers spend a lot of effort linearizing the response of their creations. Partly because analysis of highly non-linear systems like this is a royal PITA. I loved the glib statement that a negative Hooke's Law mechanism (like the first video) would stabilize oscillations. I am sure that there are conditions that would stabilize, but proving general stability seems very tough with this, particularly if you include the variability in triggering of the "cells".

author: thm_w
author_rank: Super Contributor
timestamp: May 20, 2025, 09:25:32 pm
content: The problem I see with that lead one is there is no damping. Maybe we need an advance in materials science that can provide it, then it can really take off.Here is a similar real world product that also has the same issue, it basically just ends up being a spring:https://www.cyclingabout.com/vibration-testing-lauf-grit-sl-suspension-fork-review/

author: RJSV
author_rank: Super Contributor
timestamp: May 21, 2025, 03:12:46 am
content: I bought Larry Howell's book: COMPLIANT MECHANISMSThat Mechanical Engineering book is very in depth, with tough math and models. Very serious stuff, but he discusses things like we see here, in this thread. Various latches and snap connectors.I found it too academic and math intense, to follow effectively, but that might be perfect for someone well versed in math and modeling mechanical devices.

author: coppercone2
author_rank: Super Contributor
timestamp: May 21, 2025, 08:00:54 am
content: and then someone is going to do it with a ball detent after you spend 1000 hours in CADhttps://www.amazon.com/Thread-Ressort-Detent-Socket-Plungers/dp/B0C2W5YXZM

author: ejeffrey
author_rank: Super Contributor
timestamp: May 21, 2025, 04:13:50 pm
content: Quote from: CatalinaWOW on May 18, 2025, 03:28:43 amNegative springs are somewhat commonly used in vibration isolation applications because you can get a very low resonance frequency without an insane size. For a simple spring-mass system to have a resonance frequency of 0.5 Hz, a linear spring would need to be compressed approximately 1 meter. But if you use a stiffer mechanical spring in series with a negative spring (such as an inverted pendulum) you can get an effective very low spring constant in a much smaller package. It needs to be adjusted for the operating load, but if that's OK they work pretty well. You also need damping, but you need that in any case.An example would be optical tables from minus k:https://www.minusk.com/products/mk52-optical-tables-vibration-isolation.html

author: mawyatt
author_rank: Super Contributor
timestamp: May 21, 2025, 06:08:59 pm
content: OP,Those videos reminds of the miniature moving mirror Michelson Interferrometer developed back in ~1980 for the XM21 Remote Sensing Chemical Agent Detector. This small Moving Mirror Interferrometer had no conventional bearings, neither in the mirror moving mechanism or the drive motor.The moving mirror and Voice Coil type drive motor utilized "Flex Pivots" in place on conventional bearings. These are thin blade filaments between concentric hollow tubes, where the outer tube and inner tube are only connected by the thin blade filaments radial to the tube which allows the inner tube to rotate wrt the outer tube ~45 degrees. These were utilized in the Interferrometer because of the precision, repeatability, and lack of any mechanical noise, as well as the external vibration sensitivity for Battle Field Ground use requirements.We have some Piezo Electric based Linear actuators (Physik Instrumente) for positioning chips for high resolution imaging we were involved with long ago, which use a flex type bearings. These achieve nanometer resolution and are machined from solid blocks of Stainless Steel with the Piezo Ceramic Elements wedged between "Lever Arms" that are cut into the SS block. Precision Laser cuts in the SS Block create effective flex bearings that allow the Physical Expansion of the Piezo Ceramic Elements (which occurs under applied voltage) which "bends" the SS Lever Arms, and this is translated into linear motion by the flex bearings created by the Laser cuts.Anyway, fascinating stuff indeed!!Thanks for postinghttps://www.eevblog.com/forum/projects/xm21-remote-sensing-chemical-agent-detector/Best

author: coppercone2
author_rank: Super Contributor
timestamp: May 21, 2025, 08:38:25 pm
content: is that not what the mechanism for a hard drive head is currently? The motor does coarse adjustment and then the piezos bend it to do fine adjustment?

author: coppercone2
author_rank: Super Contributor
timestamp: May 21, 2025, 08:42:51 pm
content: I wanna know something about those. If you have air capacitor at high voltage and you stick the piezo material in there on a tooth pick would it contract and expand or does it need to be metalized and directly connected to the supply? I don't see why it would need to be connected since its a non conductorCan you put some machine made out of piezo electrics in between the plates of a big capacitor and have it do stuff based on the plate charges. I have some recollections of things in electric fields for physics home work.ALso what voltage do those hard drive piezo crystal need?Does it expand in the direction of the field or does it have somethjing to do with its own crystal structure too?What if you put a piezo object in a electric field and rotate it. is it going to like have a bulge that travels with the rotation?