Letters 8/6/2009

July 30, 2009
The article on bolts (“What’s a Nice Bolt Like You Doing in a Joint Like This,” June 18) is generally good from a qualitative perspective.

The nuts and bolts of nuts and bolts
The article on bolts (“What’s a Nice Bolt Like You Doing in a Joint Like This,” June 18) is generally good from a qualitative perspective. Unfortunately that makes it fairly inapplicable to the real world. Here are a few things about bolts I learned during more than 30 years of design experience and more than 40 years of building and fixing machinery other people have designed.

Petroleum oil actually is an excellent thread lubricant for most metal fasteners. Lubricants with a solid lubricant filler certainly work better, but in the vast majority of steel nuts and bolts, engine oil is adequate and certainly beats the heck out of using nothing.

Bolted-joint integrity depends on the bolt staying tight, assuming the joint is properly designed. The article reasonably discusses keeping the joint from loosening in service. Part of keeping the joint tight is fully tightening the fastener in the first place. Torque is one method. Unfortunately, it depends on friction, which is not easy to gauge. Therefore joints should be designed to not overstress bolts, assuming minimum friction coefficients and maximum torque. That way, fasteners are tight enough to do the job at maximum friction and minimum torque. Considering that the friction factor is ±30%, the preload variation can be almost 100% from the minimum to the maximum, which the article didn’t discuss.

The article also does not mention the angle-of-turn tightening method. This method pretty much eliminates the dependence of bolt preload on the variability of friction. Preload variation is close to the tolerance on the installer’s ability to measure the angle, which can be automated using a programmable torque wrench. Hence preload variation is as low as ±15%.

Realistically and by design standards, we are driven to using secondary locking methods as described in the article. Realistically, it is the rare fastener that comes loose on my cars and my lawn mowers (and chain saws and hand tools and power tools etc.), and almost none of them have secondary locking methods. But thread-locking compounds are much more robust than your article describes. They are not generally used in aerospace due to technical arrogance and the not-invented-here syndrome, and also because it is tough to verify whether the compounds have been applied. This can be overcome by specifying fasteners with preapplied locking compound.

I think the fancy locking washers described in the article appear to be a clever method to prevent loosening. However in my two decades or more of wrenching, I have never come upon anyone who uses these. I have never seen them outside MACHINE DESIGN or my machine design textbooks.

Scott Webb

Mr. Webb provides accurate information about friction-based fastening and we agree that friction-based fasteners are adequate for many applications. However, friction-based fasteners may not prevent bolt loosening in critical, high-dynamic-load applications found in industries such as the military, rail, mining, and aviation, and products such as gas turbines and tunnel borers. The purpose of the article is to provide helpful information about various fastener methods as well as the wedge-locking method. Nord-Lock’s bolt-securing system is relatively new, and the wedge-locking design is now being taught and discussed by several standards organizations including SAE. We invite Mr. Webb or any organization representative purchasing fasteners to visit our labs and see first-hand the testing we do that supports the points in the article.

— Julie Pereyra, Business Development Manager, Nord-Lock Inc.

Leadscrew questions
I read the article on leadscrews (“Getting the Best Leadscrew for the Job,” May 21) and found it an enlightening presentation of areas for concern when using leadscrews. Thanks for publishing it.

I have to question the author about the expression for leadscrew natural frequency. This equation, for natural lateral-shaft frequency, does not appear to account for any applied axial loading. The thrust, which was addressed, is not incorporated into the natural-frequency expression. I would expect dynamic end loading to raise (for tension) or lower (for compression) the basic theoretical “zero-thrust” natural frequency. Does axial thrust have any significant effect on a leadscrew’s lateral frequency? Should it be a consideration in choosing the right leadscrew?

Another minor item: Based on the two different published equation constants, 14.03 and 4.76, the modulus of elasticity, E, used for the screw “Pcr” appears to be 29 × 106; the modulus for the screw, N, seems to be 30 × 106. If this is the case, why are two different (probably close enough) values used for the same material?

Lee Ruiz

While axial forces applied to a rotating shaft would probably result in some change of its natural frequency, for all practical purposes the given equation provides a sufficiently accurate industry-accepted method for evaluating the lowest critical speed of a steel leadscrew. A coefficient of safety of at least 80% should be applied to the calculated value

Many thanks to the reader for pointing out a presumed inconsistency in steel’s modulus of elasticity values used in the two equations. I haven’t verified the calculation of the critical speed constant myself; it is standard throughout the industry and is based on the Machinery’s Handbook “Formulas for Critical Speeds” section (on p. 199 of the handbook’s 28th ed., the formulas are stated to “apply to steel shafts having a modulus of elasticity E = 29,000,000”).

— Igor Glikin, Sr. Mechanical Engineer

I come to praise hybrids
I am writing about the review of the Cadillac Escalade Hybrid (June 18). I worked in the auto industry for a decade and spent several years working on engines, powertrains, and in-vehicle powertrain calibration, but that’s enough about me.

In the review, there is what I consider to be a counterproductive response to the fuel-economy improvement provided by the hybrid drivetrain. It is difficult to realize even a fraction of a percentage improvement in fuel economy when trying to balance Federal regulations, customer perceptions, and cost. For an improvement of 66.7% in the city and 10.5% on the highway to be dismissed as “not a lot, but a step in the right direction” is just plain wrong. Improvements on that scale should be highlighted and praised rather than indirectly criticized.

If you want to get 30 mpg, then don’t buy a vehicle the size of a bus. And if you want 50 mpg or more, press the politicians to dispense with the games and create policies that promote clean diesel and diesel hybrids. Diesel is far more “green” than most options, especially if you factor in energy savings at the refinery level. Diesel also costs less “green” in terms of dollars.

Andy Laures

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