Dear Search Engineer: I'm having problems locating vendors who furnish injection-molded plastic tooling made from spray metal. This is to make plastic parts in 500-1,000 piece lots.—B.J. in NY
Dear B.J.: Protomold (http://rbi.ims.ca/3848-511) makes the tooling and supplies the injection-molded parts in as little as five working days. They do have a size limitation on the parts. The tooling costs as little as $1,995. For an in-depth look at Protomold's process in a Design News article, go to http://rbi.ims.ca/3848-512. At these low volumes you can also look at the RIM process to mold your pieces. RIM processing requires less expensive tooling. More information can be found at http://rbi.ims.ca/3848-513.
Dear Search Engineer: I have a machine with five different safety covers that lift with the aid of nitrogen gas springs. Three of them use 100-KN springs and two of them use 200-KN springs. The springs work if the covers are raised to their maximum, but if the operator doesn't lift the covers all the way, they may fall down while the operator is under them. Is there a friction-aided device I can use with or in lieu of the gas springs that would allow the covers to remain at any opening? —R.F. in CA
Dear R.F.: Lucid Gas Springs manufactures locking gas springs, which can be locked in several positions. You should discuss the specific application to cover any safety application issues. Applying them would require adding a device to engage and release the locking pin. You can find them at http://rbi.ims.ca/3848-514. Another way to go is to modify the guards to incorporate counter balances. Finally, check out Southco's product selection of constant torque hinges (http://rbi.ims.ca/3848-515).
Dear Search Engineer: I'm looking for technology to replace the oscillating drive on an agricultural machine. Current machines use a pitman arm or "wobble box" (i.e., epicyclic Cardan drive), both of which are expensive and prone to periodic failure. Load consists of oscillating a saw-type bar of approximately 25 lb at 500 strokes per minute against constant infeed cutting force of 10 lb. —R.K. in KS
Dear R.K.: Check out a company called Rotary Actuator (http://rbi.ims.ca/3848-516), which manufactures pneumatic and hydraulic rotary actuators.
Dear Search Engineer: When an instantaneous load (compression, tension, torque, or shear) is placed on a mechanical member, I believe that the load first bears against the mass of the material at the point of contact and then propagates through it at the speed of sound in that material. Is this correct?—F.G. in CA
Dear F.G.: Not quite. The laws of physics for balance of forces and moments are true for static situations. For dynamic situations you need to use dynamic analysis. Dynamic equilibrium is what exists at all times, and needs to be understood as preceding and creating the perturbations (microstrains) that propagate at the speed of sound.