Archive for October, 2009
10/24/2009
The Mohr-Coulomb stress criterion is based on the Mohr-Coulomb theory also known as the Internal Friction theory. This criterion is used for brittle materials with different tensile and compressive properties. Brittle materials do not have a specific yield point and hence it is not recommended to use the yield strength to define the limit stress for this criterion. This theory predicts failure to occur when:
s1 ≥ sTensileLimit if s1 > 0 and s3 > 0
s3 ≥ – sCompressiveLimit if s1 < 0 and s3 < 0
s1 / sTensileLimit + s3 / sCompressiveLimit < 1 if s1 ≥ 0 and s3 ≤ 0
The factor of safety is given by:
Factor of Safety (FOS) = {s1 / sTensileLimit + s3 / sCompressiveLimit} (-1)
Posted in FEA | Tagged FEA, FEA analysis, Mohr-Coulomb, Stress Criterion | Comments Off
10/24/2009
The maximum shear stress criterion is more conservative than the von Mises stress criterion since the hexagon representing the shear stress criterion is enclosed within the ellipse representing the von Mises stress criterion. For a condition of pure shear, von Mises stress criterion predicts failure at (0.577*yield strength) whereas the shear stress criterion predicts failure at 0.5 yield strength. Actual torsion tests used to develop pure shear have shown that the von Mises stress criterion gives more accurate results than the maximum shear stress theory.
Posted in FEA, Mechanical Design, Product Design | Tagged FEA, FEA analysis, Tresca Stress Criteria, von mises | Comments Off
10/24/2009
The maximum von Mises stress criterion is based on the von Mises-Hencky theory, also known as the Shear-energy theory or the Maximum distortion energy theory.
In terms of the principal stresses s1, s2, and s3, the von Mises stress is expressed as:
svonMises = {[(s1 - s2)2 + (s2 - s3)2 + (s1 - s3)2]/2}(1/2)
The theory states that a ductile material starts to yield at a location when the von Mises stress becomes equal to the stress limit. In most cases, the yield strength is used as the stress limit. However, the software allows you to use the ultimate tensile or set your own stress limit.
svonMises ≥ slimit
Yield strength is a temperature-dependent property. This specified value of the yield strength should consider the temperature of the component. The factor of safety at a location is calculated from:
Factor of Safety (FOS) = slimit / svonMises
Posted in FEA, Mechanical Design, Product Design | Tagged FEA, stress, von mises | Comments Off
10/23/2009
Inventors can obtain three different types of patents in the United States, namely, plant patents, utility patents, and design patents. Plant patents are rare and are used to protect a new plant that the inventor has produced asexually (without using seeds). A utility patent can be used to protect the way a new technology functions and is used. A design patent protects the visual characteristics of an item. There is often confusion among inexperienced entrepreneurs and inventors regarding the differences between utility and design patent protection. It is important to understand that a design patent protects only the appearance of an article and not its structural or functional features. It is different than a utility patent because it offers no protection for the way an article works and can only protect the unique visual “look” of a new item. As such, if you are looking to protect the way your invention works, a utility patent should be pursued. The proceedings relating to granting of design patents are similar to those relating to utility patents with a few differences. A design patent has a term of 14 years from grant, and no fees are necessary to maintain a design patent in force. If upon examination it is determined that an applicant is entitled to a design patent under the law, a notice of allowance will be sent to the applicant or applicant’s attorney, or agent, calling for the payment of an issue fee. The drawing of the design patent conforms to the same rules as other drawings, but no reference characters are allowed and the drawing should clearly depict the appearance, since the drawing defines the scope of patent protection. The claims of a design patent are different from a utility patent. A utility patent has multiple claims while a design patent is limited to a single claim. The drawings of a design patent provide a visual disclosure of the claim. In light of the differences between utility and design protection, it is important that an inventor understand the limitations of a design patent. A design patent should be filed only if the appearance of an invention is important. If it is possible to change the appearance of an invention without significantly altering its function, a utility patent is more appropriate.
John Rizvi is a Registered Patent Attorney at the Fort Lauderdale based law firm of Gold & Rizvi, P.A.-The Idea Attorneys® ( http://www.ideaattorneys.com ).
Posted in Patents | Comments Off
10/21/2009
Finite Element Analysis (FEA) provides a reliable numerical technique for analyzing engineering designs. The process starts with the creation of a geometric model. Then, the program subdivides the model into small pieces of simple shapes (elements) connected at common points (nodes). The representation of a given region by a set of elements (i.e., discretization or mesh generation) is an important step in finite element analysis. Meshing the model is the heart of any FEA analysis. The choice of element type, number of elements, and density of elements depends on the geometry of the domain, the problem to be analyzed, and the degree of accuracy desired. Local mesh refinement tools are very important to have good mesh with gradual transitions between the mesh densities. One should have a finer mesh in the areas of high stress gradient to ensure accuracy of the solution.
Posted in FEA, Mechanical Design, Product Design | Tagged FEA analysis, mesh, meshing, nodes | Comments Off
10/21/2009
What are the types of nonlinearities that can occur?In linear analysis, the response of a structure is directly proportional to the load. We assume that:
- the displacements and rotations are small
- stress is directly proportional to strain
- loads maintain their original directions as the structure deforms.
However any of the convenient assumptions that are made during a linear analysis may not hold good in real life situations. For example:
- A contact area may change as the load changes
- A material may no longer exhibit an elastic behavior especially after it starts to yield and flows into the plasticity region.
- The stiffness of the structure may decrease because of buckling or the material may even fracture!
- The displacements and rotations may become too large and thus there is a need to develop equations describing the equilibrium at various intervals instead of one single configuration.
The direction and magnitude of the applied force can change in large rotation problems.
Posted in FEA, Mechanical Design, Product Design | Tagged nonlinear, nonlinear analysis, timeline | Comments Off
10/21/2009
When loads are applied to a body, the body will deform and the effect of the loads will be transmitted throughout the body. To absorb the effect of loads, the body generates internal forces and reactions at the supports to balance the applied external loads. Linear static analysis refers to the calculation of displacements, strains, and stresses under the effect of external loads.
Posted in FEA, Mechanical Design, Product Design | Comments Off
10/21/2009
When loads are applied to a body, the body will deform and the effect of the loads will be transmitted throughout the body. To absorb the effect of loads, the body generates internal forces and reactions at the supports to balance the applied external loads. Linear static analysis refers to the calculation of displacements, strains, and stresses under the effect of external loads based on two basic assumptions:
Static Assumption All loads are applied slowly and gradually until they reach their full magnitudes. After reaching their full magnitudes, loads will remain constant (time-invariant). This assumption allows us to disregard insignificant inertial and damping forces due to negligibly small accelerations and velocities. Time-invariant loads that induce considerable inertial and/or damping forces may warrant dynamic analysis. Dynamic loads change with time and, in many cases, induce considerable inertial and damping forces that cannot be neglected.
Linearity Assumption The relationship between loads and induced responses is linear. If you double the magnitude of loads, for example, the response of the model (displacements, strains, and stresses), will also double. You can assume that the linearity assumption is valid if:
- All the materials in the model comply with Hook’s law, that is stress is directly proportional to strain.
- The induced displacements are small enough to ignore the change in stiffness caused by loading.
- Boundary conditions do not vary during the application of loads. Loads must be constant in magnitude, direction, and distribution. They should not change while the model is deforming.
Posted in FEA, Mechanical Design, Product Design | Tagged dynamic loads, linearity, Static Analysis, Time-invariant | Comments Off
10/21/2009
Thermal energy transfers from one point to another through the interaction between the atoms or molecules of the matter. Conduction occurs in solids, liquids, and gasses. For example, a hot cup of coffee on your desk will eventually cool down to the room-temperature mainly by conduction from the coffee directly to the air and through the body of the cup. There is no bulk motion of matter when heat transfers by conduction.
Posted in FEA, Mechanical Design, Product Design | Tagged conduction, thermal conduction | Comments Off
10/21/2009
Convection is the heat transfer mode in which heat transfers between a solid face and an adjacent moving fluid (or gas). Convection involves the combined effects of conduction and the moving fluid. The fluid particles act as carriers of thermal energy. The rate of heat exchange between the fluid of temperature Tf and the face of a solid of area A and temperature Ts can be expressed as:
Q convection= hA(Ts-Tf)
where h is the convection heat transfer coefficient, Tf is the temperature of the fluid away from the face of the solid. The units of h are: W/m2.°C or Btu/s.in2.°F Convection processes can be divided into two main classes:
Free (Natural) Convection The motion of the fluid adjacent to a solid face is caused by the buoyancy forces induced by changes in the density of the fluid due to the presence of the solid. When a hot plate is left to cool down in the air, the particles of air adjacent to the face of the plate get warmer, their density decreases and hence they move.
Forced Convection An external means such as a fan or a pump is used to accelerate the flow of the fluid over the face of the solid. The rapid motion of the fluid particles over the face of the solid maximizes the temperature gradient and results in increasing the rate of heat exchange.
Posted in FEA, Mechanical Design, Product Design | Comments Off
10/21/2009
Thermal radiation is the thermal energy emitted by bodies in the form of electromagnetic waves because of their temperature. All bodies with temperatures above the absolute zero emit thermal energy. Because electromagnetic waves travel in a vacuum, no medium is necessary for radiation to take place. The thermal energy of the sun reaches earth by radiation. Because electromagnetic waves travel at the speed of light, radiation is the fastest heat transfer mechanism.
Posted in FEA, Mechanical Design, Product Design | Tagged radiation, thermal radiation | Comments Off
10/21/2009
In this type of analysis, we are only interested in the thermal conditions of the body when it reaches thermal equilibrium, but we are not interested in the time it takes to reach this status. The temperature of each point in the model will remain unchanged until a change occurs in the system. At equilibrium, the thermal energy entering the system is equal to the thermal energy leaving it. Generally, the only material property that is needed for steady state analysis is the thermal conductivity.
Posted in FEA, Mechanical Design, Product Design | Tagged steady state, thermal analysis | Comments Off
10/21/2009
In this type of analysis, we are interested in knowing the thermal status of the model at different instances of time. A thermos designer, for example, knows that the temperature of the fluid inside will eventually be equal to the room temperature (steady state), but he or she is interested in finding out the temperature of the fluid as a function of time. In addition to the thermal conductivity (only material property needed for steady-state thermal analysis), we also need to specify density, specific heat, initial temperature profile, and the period of time for which solutions are desired.
Posted in FEA, Mechanical Design, Product Design | Tagged thermal analysis, Transient | Comments Off
10/21/2009
A change in the angle of inclination of a belt conveyor where the center of the curve is below the conveyor.
Posted in CAD Drafting, Mechanical Design, Product Design | Comments Off
10/21/2009
A capacity rating of a vehicle, axle-load, or livestock scale, specified by the manufacturer, defining the maximum load concentration for which the weigh bridge is designed. In the case of vehicle and axle-load scales, it is the maximum axle-load concentration (for a group of two axles with a center line spaced 4 feet apart and an axle widthof 8 feet) for which the weigh bridge is designed asspecified by the manufacturer. The concentrated load capacity rating is for both test and use.
Posted in Mechanical Design, Product Design | Tagged Car design, Load, weigh bridge | Comments Off
10/05/2009
A number suffixed to a drawing number to identify individual parts or assembliesdepicted and controlled by the drawing.
Posted in CAD Drafting, Solidworks 2D Drawing | Tagged Dash Number | Comments Off
10/05/2009
An aggregation of hardware, firmware, software or any of its discrete portions whichsatisfies an end use function and is designated for configuration management. CIs may vary widely incomplexity, size and type; from an aircraft, ship or electronic system to a test meter or round of ammunition.During the development and manufacture of the initial (prototype) production configuration, CIs are those itemswhose performance parameters and physical characteristics must be separately defined (specified) andcontrolled to provide management insight needed to achieve the overall end use function and performance. Anyitem required for logistic support and is designated for separate procurement is a CI. (MIL-STD-973)
Posted in CAD Drafting, Solidworks 2D Drawing | Tagged Configuration, Item | Comments Off
10/05/2009
The selection of the documents to comprise the baseline for the systemsand CIs involved, and the numbers and other identifiers affixed to the items and documents. The approveddocuments that identify and define the item’s functional and physical characteristics in the form of specifications,drawings, associated lists, interface control documents, and documents referenced therein. The configurationidentification is developed and maintained through three distinct evolutionary increasing levels of detail, eachused for establishing a specific baseline. (MIL-STD-973)
Posted in CAD Drafting, Solidworks 2D Drawing | Tagged Configuration, Identification, CI | Comments Off
10/05/2009
The ACI is the technical documentation governing andspecifying the performance, physical and interface requirements for CIs that are part of a higher level CI, typicallya system; this documentation is in the form of specifications, drawings and associated lists, and documentsreferenced therein, and is usually prepared during the validation phase. If there is no validation phase, the ACImay be prepared during the initial part of the full-scale development phase. Like the FCI, the ACI included testprovisions to assure all of the specified requirements are achieved by the developed CI.
Posted in CAD Drafting, Solidworks 2D Drawing | Tagged Configuration, Identification, ACI | Comments Off
3dcadware
fredvierheller
bob92003
Deepak Gupta