We describe how to obtain the constraint equations for a two point pivot and three point pivot. Designing a mechanism which can obtain a desired set of constraints is often an important step in kinematic or exact constraint machine design.
We begin with the simple lever mechanism shown in the figure below constraining the motion of two points A and C using the pivot at O.
Continue reading Writing Constraint Equations for Two-Point and Three-Point Mounts
Beams are often used in precision engineering applications. One common question is “what are the optimal support locations for a beam?” The answer depends on the desired objective. Below we describe some of the most common support locations: Airy points, Bessel points, minimum deflection, and nodal points. It turns out that these points are relatively close to each other for the uniform beam. The basic problem is sketched in the figure below. A uniform beam is supported on two points and the objective is the determine the placement of the supports in the presence of gravity.
Commonly, we need to save results from an Ansys Workbench study as a text file for post-processing in another program, such as Excel. One can right-click on a desired result and use Export, but that can be tedious if there a lot of results to save. With a snippet one inserts a Commands (APDL) object in the solution and writes APDL code to perform the desired functions.
There is a neat correspondence between the mathematical concept of a constraint and the practice of exact-constraint or kinematic machine design. This leads to some really useful insight for people designing machines with dynamics in mind. The PDF below is the extended abstract for a paper that Justin presented in Austin last week at the ASPE annual meeting.
Continue reading Design of Constraint Systems Based on Minimax Properties of Eigenvalues
For a bonded contact, which is the default for models opened in Ansys Workbench Mechanical, the contacts are modeled using elements TARGE170 and CONTA174. The thermal conductivity and stiffness of the contact elements are calculated by Ansys based on the properties of the two bodies. The controls only allow one to change the contact stiffness factor FKN which is a multiplier but not the actual stiffness of the joint (ie with SI units of N/m/m^2). However if one would like to set the value of the contact stiffness or thermal contact resistance, it can be done using Command snippets (also known as the Commands Object) for each contact. Continue reading Setting Mechanical Contact Stiffness and Thermal Contact Conductivity Values in Ansys Workbench using Command Snippets
This entry discusses different definitions of CTE, their relation to thermal strain, how to convert between the different forms, and how to use them in a model. The forms discussed below include instantaneous coefficient of thermal expansion (CTE), secant coefficient of thermal expansion, and direct use of a thermal strain function.
Continue reading Thermal Expansion: CTE Definitions and Thermal Strain
Thin layers of adhesive, plastic, or rubber are often employed in precision machines for joining, shimming, and sealing. These layers are often the most compliant and most dimensionally unstable elements of an assembly, so it is important to understand their behavior.
Continue reading Expansion and Stiffness of Thin Adhesive or Rubber Layers
Microslip in rolling motion is often very complicated, but the net effect can sometimes be estimated pretty easily based on strain and the resulting changes in velocity.
The power spectral density (PSD) is one of the primary ways we characterize random or broadband signals. In many cases, a PSD is read from a signal analyzer and used qualitatively to describe the frequency content of a signal. But to do anything quantitative with a PSD, we need to understand its units.
Continue reading Units of Power Spectral Density
Most people interested in machines know that adding a slight crown to a pulley tends to keep flat belts centered. But it is not obvious how it works.
