Lifetime & reliability

Oct. 1, 2004
Motion System Design turns to industry experts for some tips on how to get long life and high reliability from a design.

Who’s who

Charles Manning, Omron Electronics LLC, Schaumburg, Ill.
Andy O’Connell, V.P. Operation/Engineering, Rockford Ball Screw Co., Rockford, Ill.

Define lifetime and reliability. What’s considered leading edge today?

MANNING/OMRON: Historically, lifetime has meant decades of service for mechanical and some electrical designs. Reliability has meant long life and accurate, service-free performance.

Today, product specifications seem to change with the latest technology. Now, a lifetime is measured in “Internet time” and may last a year or less, particularly with respect to processors. Reliability is still important while a product is considered current, but replacing that product with a newer technology is acceptable as ownership and purchase costs are reduced. Some users can accept a shorter product life span in exchange for a superseding technology at a lower price.

O’CONNELL/ROCKFORD: Lifetime and reliability for ball screws generally is defined as the effective linear travel that 90% of a group of ball screw assemblies will complete before the first sign of fatigue develops. This is equivalent to an L10 life rating similar to bearing ratings. Even though the standard states that 10% may not make the rated life, probability says that 50% will surpass this life (up to 5x the life).

In what applications are lifetime and reliability most important?

O’CONNELL/ROCKFORD: The applications that require long life and high reliability are any that rely on the drive system to accomplish the design life. Prime examples of high reliability for ball screws would be in airplane wing flap actuators and nuclear vibration dampers.

MANNING/OMRON: Lifetime and reliability are important considerations for every application. Some may be more critical with respect to safety, loss of product, or profit loss. Consider oil or gas drilling, where downtime may cost tens of thousands of dollars per day, or even per hour. Reliable performance is mandatory, and users can easily weigh the added costs of redundancy or oversizing, in the interest of guaranteeing uptime. Even though all applications are not created equal, it’s both good engineering and a good business practice to offer users a solution that will perform reliably for as long as possible.

What is the biggest limiting factor in a motion system when it comes to lifetime and reliability?

MANNING/OMRON: Unfortunately, it’s not necessarily a specific mechanical or electrical component. Many times, the purchase cost seems to be a limiting factor. Users should consider overall ownership costs, including purchase price, installation costs, added productivity, and product reliability before making a purchase to ensure the best solution is applied.

O’CONNELL/ROCKFORD: The biggest limiting factor in achieving long life for ball screws and lead screws is, first and foremost, proper screw size for the application. Other factors that affect reliability are lubrication, contamination, and improper installation.

What should engineers know about component interactions, good or bad, and how they affect lifetime and reliability?

O’CONNELL/ROCKFORD: The rated life of similar ball screws can vary from manufacturer to manufacturer. Generally, screws manufactured in the United States are rated for one million inches of life, whereas many overseas manufacturers rate their screws on one million revolutions of life.

For instance, on a screw with a 0.2-in. (5 T.P.I.) lead with the same load rating, the screw rated on revolutions will have one-fifth the life of one rated on inches of life. Thus it may appear that the screws rated on revolutions of life have higher loads than identical screws rated on inches of life. This is an important difference to keep in mind when comparing seemingly identical screws from different manufacturers.

MANNING/OMRON: Engineers should take advantage of a manufacturer’s expertise about its product. Ask questions, and get a proof of concept if necessary. A poorly combined system will require tinkering at setup, and beyond that, may negate any cost or individual performance benefits. Consider using a family of products from a single manufacturer where it’s practical, or ask a manufacturer for recommendations about potential vendors for accompanying products. For example, a vendor that can provide a complete automation solution is able to certify that its PLC, drive, sensor, and temperature controller will work together out of the box, which reduces the cost of ownership.

What tips can you offer to help engineers optimize lifetime and reliability in their designs?

O’CONNELL/ROCKFORD: To achieve the maximum reliability from your ball screw application, proper sizing during the design stage is critical. All loading characteristics need to be taken into account. Off-center or moment loading of the ball screw will reduce assembly life. In addition, if the loading varies during screw travel, an equivalent load that is less than the maximum load can be used for life calculations.

MANNING/OMRON: Always take advantage of the expertise of a manufacturer during the design stage. Lean on the people that know the most about a product, and let them offer advice about pitfalls, failure points, or secret tricks that can extend application life. Good manufacturers store information about applications, and users can use existing knowledge to their advantage. Also consider asking other engineers. Use the Internet and find a Web site where visitors are encouraged to post questions and responses. Many times these sites are sponsored or hosted by the magazines that we all read every month, so check your favorite search engine.

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