High Speed Magnetic Rotary Encoder

Primarily made by Austrian MicroSystems, these encoders use an array of hall effect sensors to accurately sense the angular position of a shaft (or anything that rotates with a magnet). This technology is cheap, continuous, and provides an absolute position – meaning that the motor controller has much better information to work with when magnetizing the windings.

AMSencoder.jpg

Figure 1: from directindustry.com

 

Some example product details: AS5134_Factsheet_v1_7, http://ams.com/eng/Products/Magnetic-Position-Sensors

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High Speed Magnetic Rotary Encoder

Belleville Springs

Belleville springs are conical washers that rely on the curvature of the shell to provide stiffness as it is compressed.

Bi-stable: Their geometry means that belleville spring can have a bi-stable snap-through behavior. This behavior can be very useful in mechanisms that we want to fail in a stable position, like this poppet valve: http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20090016268.pdf

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From Fundamentals of Machine Elements by Schmid, Hamrock, and Jacobson

 

Hysteresis: Unlike helical springs, Belleville springs also generate friction as they deform because there is a coupled radial and axial motion at the outer and inner rim. This friction creates hysteresis in the spring’s force-deflection curve that (although it can inhibit repeatability) can also act as a damper if that is needed.

Figure 12. Isothermal load–deflection curve of the Belleville spring at T t = 323 K and subsequent thermal cycle. 

From NiTi Belleville washers: Design, manufacturing and testing, by Maletta et al.

 

Compact: Due to the non-linear nature of Belleville springs, it is possible to get a very large stiffness in a small volume. Additionally, Belleville springs can be stacked in series (like a bellows) to reduce their stiffness, or in parallel to increase their stiffness.

cat_belleville_pic2.gif

From Raymondasia

Information also taken from: https://en.wikipedia.org/wiki/Belleville_washer

Belleville Springs

Vicsous Shaft Couplings

Most automatic car transmissions use a fluidic coupling to transmit torque. Fluidic couplings have the benefit of controlling the relative speed of two shafts with the shear forces generated in the working fluid (usually Silicone) between a set of discs and the housing. In automatic transmissions, these couplings allow one shaft to spin freely for a brief period of time while gears are changed.

In the case of extreme slip, the fluid heats up and thermally expands. This expansion pushes the discs against each other, nonlinearly increasing their ability to transfer high torques. This self help, called the hump effect, also prevents the coupling from overheating. When the fluid expands under heat, the shafts spin at nearly (if not the exact) same speed, reducing the generation of heat from shear in the fluid.

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image and concept taken from Mark’s Standard Handbook for Mechanical Engineers

Vicsous Shaft Couplings

Adjustable Keys (p-set 3)

If you have every tried to attach something to a shaft before, you know that in the cost-quality-performance triangle of project management, there is not much winning.

  • Interference fits require careful design and high tolerance machining, plus they can fail under thermal expansion or centripetal acceleration.
  • Pins can induce stress concentrations in the joint, and often require a pin and hole with high tolerances. Plus access to the pin slot can be problematic
  • Keys require broaching and also create (albeit smaller) stress concentrations
  • Collar clamps can be bulky and hard to integrate into the machine, plus they require compliance
  • D-shafts can strip out, and require special machining
  • Don’t even get me started in set screws

Each of these joinery methods have advantages too, albeit some have more than others. However, making these joints is often a big part of machine design that can be easily mistaken. So, I decided to go out and see if there are other design elements, lego bricks so-to-speak, that could be added to the list.

Because there are have been many talented engineers working on hard problems throughout history, it did not take me long to find another great way of solving the shaft joinery problem that someone else had thought of: adjusting keys. I found the concept in a book titled Textbook for Vocational Training − Machine Elements and Assemblies and Their Installation by Früngel, Geppert, and Steckling.

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A simple adjusting key assembly, from Textbook for Vocational Training.

The assembly works by splitting the shaft collar into two halves, and putting an angle on the back of one of those halves. A key, with a matching angle is inserted into a groove with the angled shaft collar, and held in place with friction, a bolt, or something else depending on the design constraints. At the cost of part count (adding 2-3 components), the design is made less complicated by making the parts out of easily manufacturable geometries. The orientation of the key angle can also be reoriented to suit the application! An excellent tool to keep in the design engineer’s toolbox.

Adjustable Keys (p-set 3)

Shear Pin Loading and Failure (p-set 1)

Note: This is this weeks Seek and Geek

I was reviewing my calculations for the strength of a pin in shear, and was finding unsatisfactory reference. Most free-body-diagrams (FBDs) for shear pins do not enforce conservation of momentum. All drawings would begin spinning instead of remaining static. Any FBD that included conservation of momentum did not calculate the bending stresses induced by the internal moment and factor for how that extra stress affects the strength of the pin.

So, I assumed that there is a small amount of slop in the pin causing the pin to contact the joint in small patches. The length of those patches is D/5 (1/5 the diameter of the pin) as a simple St. Venant estimation. Applying a constant distributed load over those patches and applying conservation of linear and rotational momentum on the pin and each handle gives a failure point that is slightly offset from the center of the pin and with an applied force that is 85% the magnitude of a predicted failure force just looking at shear alone. The full calculation is shown in paper below, and will be revised to a cleaner format, such as latex.

PinAnalysis1a

 

PinAnalysis2

Shear Pin Loading and Failure (p-set 1)