Ask The Search Engineer

Dear Search Engineer: Am I nuts or is a calculated torque value the same whether you apply the torque to a bolt head or to a nut? In other words, if we have a bolt assembled into a blind, tapped hole, would the torque value be the same as if it was a bolt with a nut joint? —M.R.in Indianapolis

Dear M.R.: The torque values for friction in the threads would be the same, regardless which is turning. However, the torque due to friction under the bolt head when it's being turned may be lower than the torque from the friction under the nut face when the nut is being turned. Both the bolt and nut have annular areas at their faces that contact the joint. The annular surface area on most nut faces has a radius greater than the average radius of the annular face under the bolt head. Therefore, with the larger radius, a given friction force due to clamping load will cause a higher torque when turning the nut. Torque due to friction in threads and at contacting faces is often very significant and is also the most difficult portion of total torque to estimate. And don't forget that the friction torque is also affected by material and lubrication.

Here's one for you, Search Engineer: We just upgraded to AutoCAD 2002, and I need a way to create revision clouds. Is there a way to do this? —B. S. from L.A.

Dear B.S.: The Revision Cloud Bonus utility is not included with the 2002 version of AutoCAD. But if you find your older version of AutoCAD and copy the "revcloud.lsp" file to the SUPPORT folder in your current version of AutoCAD, the cloud issue will clear up.

Yo, S.E.: I need a recommendation for a finish for phosphorus bronze electrical contacts that will maintain brightness. Ideally, the finish will not dramatically affect conductivity, and will survive some handling during final assembly. The brightness is mainly for aesthetics. —P.E. from Newark

Dear P.E.: The best plating combination for a phosphorous bronze or beryllium copper contact is first a copper strike then a nickel diffusion barrier (50 micro inches) then gold (10 to 50 micro inches). Gold has the lowest contact resistance but nickel is not too bad for higher current applications. Silver has low contact resistance, but it tarnishes and loses its good looks after a while. Gold and silver are very porous, so you need a diffusion barrier. Nickel is very brittle, so you must plate after forming the contact.

Dear Search Engineer: I have an application requiring an extremely "springy" material. The particular property of importance is the dissipative loss or hysteresis, which needs to be minimal. In other words, the ideal material would "ring" indefinitely when struck or plucked. —J.A. from Huntsville, AL

Dear J.A.: A cobalt-nickel alloy is a very good spring material. It has high strength, ductility, excellent fatigue life, and is non-magnetic and suitable for high temperature application.

Dear Search Engineer: I work for a system integrator and we are doing a quote for a customer where we will have to laser mark a serial number on a part made out of POM. Can this be laser-marked? —J.C. in North Carolina

Thanks for asking J.C.: Actually POM is polyacetal, a highly crystalline engineering thermoplastic ideal for parts requiring durability, dimensional stability, and low wear. There are two basic types—homopolymer like Delrin^®, and copolymer, such as BASF Ultraform. And yes, POM can be laser-marked. But there are a couple things to keep in mind for successful laser-marking of POM or other thermoplastics: A homogeneous distribution of absorbing pigment is necessary; therefore, using a precolored resin is generally better than "salt and pepper" coloring or coloring at the machine. And keep in mind that if the base resin contains other fillers or modifiers, this can affect laser markability. CO2 lasers will give a 3D etch into the resin where Nd-YAG laser will potentially yield a white or other color contrast. Good luck!

Dear Search Engineer: I have a question concerning the amount of thread required to protrude past a fastener. I remember that the length of thread that must protrude is one half the diameter of the bolt size. Is this correct? —D.S. from Seattle

Dear D.S.: Usual shop standard is for two full threads to extend past the nut or nylon insert.

Dear Mr. Search Engineer: We control the throttle on a diesel engine with a linear actuator. The device is currently mounted on the side of the engine and experiences all the vibrations associated with a diesel engine. Heat and dirt are also present as the engine is inside a sound enclosure mounted on an off-road vehicle. Input comes from a DPDT switch in the operator cab. Force to control the throttle is small, less than 10 lbs. Current selection fails regularly and warrantee claims are climbing. Is there a better way to provide rugged, low-cost, fail-safe throttle control? —R.P. from Austin, TX

Howdy R.P.: Can I assume that by fail-safe you mean that upon loss of signal the throttle remains in its current position? I am also assuming that because you are using a DPDT switch to control the throttle that one DPDT switch position causes the linear actuator to open the throttle until a limit switch is made and the other DPDT switch position returns the throttle to idle, again with the aid of a limit switch. If you would consider fail-safe to mean that upon loss of signal the throttle returns to idle or at worst remains in its current position, I would suggest that you explore the possibility of using a pneumatic cylinder for an actuator, either controlled by an air valve switch in the operator cab or by the current DPDT switch through a solenoid operated air valve. The air cylinder could be operated between adjustable mechanical stops to provide the idle and fast throttle operating position. Most diesel vehicles utilize compressed air for the brake system and it would not do harm to draw a small quantity of air from the system to operate a small air cylinder. I would suggest using a restriction (orifice) at the fitting where you tap the air to limit air loss to an acceptable level in the event that the air line to this type of throttle controller breaks. For a system that returns to idle upon loss of signal, the cylinder would be pressurized to throttle the engine up and the air exhausted from the cylinder so that the throttle return spring can return the engine to idle. Alternately, each end of the cylinder could be pressurized and exhausted as appropriate to force the cylinder to move, in order to throttle the engine up and down. If an air cylinder is used, it must be carefully selected for the environment in which it will be used with respect to temperature and moisture. Also, it must be designed to operating without externally applied lubrication.

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