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Mechanical Engineering Exchange

Q&A Forum
As experts in structures, heat transfer, fluid networks and piping, machine design, process simulation modeling, process improvement and reliability among other mechanical engineering areas, our consultants offer their advice. If you have a question in one of these areas, or any area discussed on our website, send an email to us.
All relevant ME Questions and Answers are posted below.
 
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Q&A Postings (We will post relevant Q&A's as we receive them.)
 
Lubricants   
   What is the effective ingredient in food-grade mold release and will it work to separate components that become stuck through a vacuum?

Most industrial food-grade mold release agents contain silicone oil in amounts varying from >0.1% to 3%. A very small number of food-grade release products contain soy bean oil primarily for the release of industrial baking equipment from the food product. The silicone oil can withstand a higher temperature than soy oil and does not brown and become sticky after repeated heat applications. For these reasons, silicone oil is more practical, although more expensive (at between $26-$52/case of aerosol cans) for most food release applications.

The silicone oil is, in general, inert and can be used in contact with food (as regulated by the FDA) as well as other materials. The oil serves to lubricate the contact surfaces and allow separation when one surface slides against the other as during mold cool-down. When combined with some force from mold ejection pins, the food product easy separates from the the mold surface.

However, when two parts coated with silicone oil are stuck together by a vacuum, the silicone oil promotes sliding but not a release from the vacuum inless the parts are slid completely apart. Usually, the vacuum must be released by either prying the parts or by lowering / eliminating the vacuum pressure between them.

   
   Is grease better than oil for ball or roller bearings?

For rolling element bearings an oil may be preferable as viscous dissipation (hence, traction torque) would be minimized. For high speed applications (~100,000 DN) grease can not be used. The downside is that for a non-recirculating system the oil would have to be replaced more frequently. Consideration must also be given to additives such as oxidation inhibitors, EP (extreme pressure) additives, etc., as these may be required for specific applications. For example, EP additives are typically added to 80W-90 oils used with taper roller bearings having high edge loads as is found in a gearbox applications.

Greases are preferred where maintainence requirements are to be held to a minimum and where a re-circulating oil system is not feasible. Further, grease packed bearings tend to be more tolerant of contamination.

The application has to be defined. For example, a car wheel bearing is a low speed application but is always grease packed since it isn't routinely serviced. Overall, for low speed applications, except for inline skating, grease is almost always used because it requires low maintenance and no oil supply system.

   
Bearings / Transmission Components  
   
Heat Transfer  
   When should an FEA be done v. hand calcs? In general, FEAs should always be performed when geometries are complex and the results allow little margin for error. Hand calculations are well suited for 2D results, simple calculational iterations, and answers that can have safety factors applied to deal with calculational simplifications and inherent uncertainties. However, the results of all FEA analyses should be checked either through some sort of simplified hand calculation or through the use of another FEA program and another meshing arrangement, and perhaps, through the use of other FEA elements.
   
Fluid Networks / Piping  
   How much importance should be placed on corrosion resistance v. corrosion inhibitors?

The effects of corrosion are very important and should be considered for every piping network design. Depending on the purpose of the network and the importance of maintaining cleanliness of the fluid, either corrosion resistant piping or corrosion inhibitors should be used to maintain the integrity of the pipe pressure boundary. For example, networks designed to convey and contact food or other fluids that cannot be contaminated require corrosion resistant piping to prevent or limit foreign (unknown or undesirable) chemicals from leaching into and being absorbed by the fluid stream. Networks designed to contain nonfood products and fluids whose chemical composition can change over time are strong candidates for the addition of corrosion inhibitors to the fluid to prevent pipe corrosion.

There are cost considerations for selecting one mode over the other. Normally, corrosion resistant piping will be significantly more expensive (ranging from 1.2X - 8X higher) than using carbon steel, iron, or common plastics with corrosion inhibitors. However, corrosion inhibitors require procedures for safe handling, disposal, and periodic testing to maintain effectiveness over time. Some inhibitors can also pose dangers to personnel as the chemicals evaporate and are released from the network to the atmosphere.

In either case, the designer must choose a mode for corrosion control or design extra strength and longevity into the system. And because different piping materials react differently to different liquid environments (stainless may not be the best for all environments) it is important to consult with a qualified materials and corrosion expert.

   
Structural  
  Can I get a solution to an indeterminant [truss] problem? In an indeterminant problem, there are not enough equations to solve for all the desired unkowns. So, we must find another equation or consider the system to be unstable. If the system is unstable, then additional stabilizers need to be added.
   
Process Modeling and Improvement  
   My machine processes product fine on first shift and by the end of second shift produces greater than 10% defective product. What can I do to fix this? Technicians are probably adjusting various machine controls to influence certain features of the product and are in turn inadvertently degrading other product features. The best approach is to make a chart containing a list of the features to control, the range of acceptable measured values for each feature and the process control settings and ranges for each acceptable range of feature values. The only way to effectively create this chart is to conduct a DOE (Design of Experiments). In every one of these cases, technicians had a "gut-feel" for the direct impact of an important process control on the product features but did not know the interdependency on the other features. A chart created from DOE experiments will indicate the interdependencies and provide a plan for adjustment. A DOE used in this way is often called "process centerlining" because it clearly identifies the acceptable range and best-fit machine adjustments required for an optimized process.
   
Materials  
   Copper C8110: Where can I find a stress/strain curve? Try www.matweb.com. This copper casting is listed with material properties. Check with the listed manufacturer for the stress-strain curves.
   
 

 


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