How to get the most from customized leadscrews

Oct. 6, 2011
Start with ideal specifications and keep an open mind

Authored by:
Tom Solon, PE
Engineering Manager
Haydon Kerk Motion Solutions
Hollis, N. H.
Edited by Jessica Shapiro
[email protected]
Key points:
• Custom-designed leadscrew assemblies don’t necessarily cost more or have longer lead times.
• Detailed performance specifications are a good starting point for talking with leadscrew manufacturers about design.
• Consolidating functions into fewer custom-designed parts can simplify assembly, cut maintenance, and even lower part cost.
Haydon Kerk
“Don’t Settle for Standard Parts,” Machine Design, June 23, 2011
“Introduction to Linear Actuators,” Webinar hosted by Haydon Kerk and Machine Design, Nov. 13, 2008

Some companies offer a few options on a device and say they have custom products; unlike Henry Ford’s dictum: “any color as long as it’s black,” they also offer white and red. But true custom components are limited only by imagination and manufacturing technology.

This is particularly true for leadscrews. Engineers turn to these devices when accuracy and reliability are critical. And custom leadscrews make sense whenever price and cost of ownership matter.

What if ...
From a manufacturer’s perspective, custom leadscrews are those built to customer specifications rather than those of the manufacturer. A true custom supplier challenges engineering customers to answer “What if …” questions to define exactly what they want.

For instance: If cost was no object, what would the perfect component look like? What would the motion profile be in the absence of friction? What if the screw could be as long or short as needed? And what if the nut could be any shape or size? How could the device perform better? How could we simplify assembly or use fewer parts?

The keys to success are imagination and avoiding constraints at the outset. Both are often hindered by common misconceptions.

Many believe custom parts always cost more. In fact, the ability to combine components and cut assembly and maintenance may make custom parts more affordable. An assembly of specialized parts might outperform an actuator comprising an integrated motor, leadscrew, and nut in some applications, but considering a custom design that takes all application factors and costs into account often has better overall cost of ownership.

Custom parts are also thought to have long lead times. But manufacturers of custom parts can advise engineers on processes that can meet schedule restraints. For example, many engineers mistakenly believe injection molding only makes sense for large quantities due to the complexity of the mold, but this is no longer always the case.

Likewise, the technology for prototyping parts has advanced to the point where it is not necessarily cost prohibitive and may actually save money in the long run.

By talking over the application with a manufacturer, engineers can quickly assess the supplier’s ability to provide custom parts. At the outset, it’s important to avoid the urge to rule out options. No supplier is going to offer a design direction that increases his own risk of failing.

How to go custom
Many engineers accustomed to the constraints of standard leadscrews find exploiting the benefits of custom alternatives challenging. For most engineers not familiar with the range of customization possibilities, the best place to start is with performance specifications including motion, environment, and location. Business considerations such as life cycle, production schedule, and budget also come into play. (See “Custom leadscrew checklist.”)

Motion. How will the component move? Specifically, what distance will it cover and in what direction? How fast should it cover that distance? What is the frequency of travel? How precise does the motion or final position need to be? Finally, how much load will be moved?

Environment. What conditions will the component work under? What are the expected maximum, minimum, and average temperatures? Will it encounter radiation, moisture, or chemicals that require careful materials selection? How about vacuum or pressure? Are there cleanliness standards that must be met? What contaminants, if any, are expected?

Location. Where is the component going to be? How much room is available? What support structures and mounting provisions are available? How visible or hidden should it be to operators or customers? Does it need to mate to other components?

Life cycle. How many cycles should it complete between maintenance intervals and over its lifetime? Alternately, what period of time is it expected to last?
Production schedule. How many are required? When are the first and last components needed?

Budget. What is an acceptable unit price for the component? What start-up and tooling costs are acceptable given the length of the production run?

After defining as many of these parameters as possible, an engineer should intelligently choose component specifications. For instance, quantity, budget, and timing will influence manufacturing methods and tooling investment.

Analyzing speed, loading, and duty cycle will establish the minimum PV — the product of load and velocity — the component needs. It will also help the engineer determine the acceptable friction. Both PV and friction affect material selection. Material choices also depend on corrosion risk and chemical and thermal compatibility.

Open options
Manufacturers customize leadscrews by specially machining screws, nuts, and other components. They can make the parts with user-defined materials or processes. And they can incorporate guide features or ensure seamless integration with downstream components like motors.

The screws themselves can be machined to accommodate bearings, couplings, pulleys, and other attachments. Custom machining also lets designers add special leads or specify thread forms that work best for their motion profiles.

The manufacturer can also machine custom-mounting features into leadscrew nuts, size the nuts to fit tight space, or incorporate performance enhancements such as compliant mounts or vibration damping into the nuts. In some cases, injection molding nuts from powdered metals or plastics gives manufacturers the flexibility to add features like encoder flags, points, metal housings, and mounting brackets to the nuts.

Custom design also opens up a range of materials for both screws and nuts. Leadscrews are commonly made from stainless steels and nuts are often acetal or bronze. But screws can also be aluminum, titanium, or specialty steels, and nuts can be made from carbon-fiber composites; engineering polymers like fluoropolymers, polyetheretherketone (PEEK), and polyamide-imide; or a wide variety of bearing-grade metals and polymers.

Leadscrew manufacturers offering customer-specific designs usually work with designers to incorporate guide features. Some designs will take into account the relationship to external guide mechanisms like slides or rails. Some will actively engage the guide.

Consolidation adds value
An important step on the path to custom leadscrews is to evaluate overall assembly. This means looking at the components connected to the leadscrew. For instance, it is often possible to turn the components attached to the nut into the nut itself. And combining motors and guides saves space, reduces component count, simplifies assembly, and lowers total cost.

To look at where consolidating a design makes sense, an engineer should ask:
• How will the screw be supported?
• How is the screw driven?
• What attaches to the nut?
• Can the leadscrew assembly work as a structural element?

Answering these questions can reveal new opportunities. For example, rather than attaching a nut to a carriage, the carriage, nut, guide bushings, and sensor flags can all be a single component. If a design calls for a timing pulley or gear, the nut can be designed to perform these functions which can make assembly and maintenance easier and improve reliability.

The entire leadscrew assembly — leadscrew, nut, bearings, coupling, and rotary motor — can be converted to a motorized axis. This can be done by attaching the leadscrew directly to the motor’s rotor or integrating the nut into the rotor and passing the leadscrew through the motor’s center.

A custom leadscrew assembly can even incorporate the entire guiding structure. For instance, Haydon Kerk’s ScrewRails, RGS, LRS, and MotoSplines run guide rails concentrically around the leadscrews.

And while standard leadscrew assemblies rarely carry structural loads, custom designs can. Using a leadscrew as a stressed element can simplify frame structures.

Not only do consolidation strategies reduce part counts and assembly times, they can also cut individual component costs. Compared to component assemblies, these integrated solutions eliminate the need for couplings and duplicate bearings.

Once these duplicate bearings are out of the design, mounts can be simpler because they no longer need to precisely support the eliminated bearings. The structural framework previously needed to ensure alignment of individual components can then be replaced with less precise — and less expensive — sheet-metal or polymer elements.

And once nuts incorporate simplified attachment points, sensor mounts, and guide bushing replacements, they can eliminate carriages that required complex machining.

The supply side
Even when engineers specify leadscrews with custom parts in mind from the get-go, they still might have misgivings about cost effectiveness and delivery schedules. Many designers base these opinions on experience, and it would be misleading to suggest all manufacturers are equally positioned to provide custom leadscrews.

However, it would be equally misleading to suggest that engineers have to pay a penalty to get custom parts. CAD/CAM, CNC machining, rapid prototyping, and lean manufacturing let suppliers use the same manufacturing processes for moderate runs of custom products as for huge standard-part runs.

By asking the following simple questions, engineers can get a feel for whether their chosen suppliers understand true custom design and have the ability to deliver.
• Does the company actively promote customized leadscrews?
• Does the company have strong applications-engineering support?
• Are lead times for custom parts similar to those for standard ones?
• Does the company actually manufacture parts instead of repackaging components available from other firms?
• Do company representatives pay attention to performance specifications instead of simply assigning part numbers?

Each company has its strengths and weaknesses, but all contenders will exhibit similar approaches to defining custom products. The right manufacturing partner can put a custom-design project on the road to both high performance and good value without the need to settle for off-the-shelf parts.

© 2011 Penton Media, Inc.

About the Author

Jessica Shapiro

Jessica serves as Associate Editor - 3 years service, M.S. Mechanical Engineering, Drexel University.

Work experience: Materials engineer, The Boeing Company; Primary editor for mechanical and fastening & joining.

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