Rollon Linear Guides

Lay flat seats make the skies friendlier with some help from the right linear guides

If you’ve been lucky enough to fly business or first class on an overseas trip, you may have noticed just how complex the seats have become from a linear motion standpoint. The most advanced seats now fold completely flat and offer numerous neck, back, and leg adjustments.

These super seats, which consist of up to 5,000 parts and cost more than $250,000 each, can incorporate as many as 8 different linear guides. A couple of technical key factors have emerged as the most important when selecting linear guides for seating applications.

The Right Weight. Notice we said “right” and not “light.” It’s an important distinction in this application. Thanks to its strength-to-weight ratio, titanium has traditionally been perceived as the ideal material for interior structural parts. Sourcing difficulties and high costs, however, have made it less and less attractive in seats. Increasingly, steel linear guides get the nod in seat applications.

Steel linear guides, many of which are off-the-shelf components, are widely available and offer a comparable strength to titanium at roughly one-fourth the cost. Steel does weigh roughly twice as much as titanium. Yet steel’s weight penalty is minimal in the context of a super seat. On a 300-lb seat, for example, the steel rails represent less than 1% of the total weight.

Long-term Reliability. Aircraft seats have to be robust to pass the industry’s extreme testing regimen and hold up to years of passenger abuse. The airline’s expectation of these seats is that they will last for 5 to 7 years with few maintenance issues.

As the seats rely more heavily on linear motion, one of the growing threats to seat reliability is linear guide misalignment. Misalignment produces excess friction that can makes seats difficult to operate and, ultimately, wear out before their expected lifecycle.

Traditionally, seat engineers have addressed misalignment problems during the assembly process. They specified rails and bearings separately, leaving the alignment up to assembly technicians. However, this approach is time consuming, costly and error prone.

An alternative approach has emerged recently. Rather than “assemble-in” the guide alignment, seat engineers are instead picking self-aligning, preloaded linear guides. Our Compact Rail system, for example, can absorb rotational positioning errors, misalignments between lateral planes and longitudinal parallelism errors — all while maintaining the original preload setting.

Compact Rail linear guide systems have additional benefits in seat applications, including noise reduction and a high dynamic load capacity. For more detailed information, download our white paper on linear motion for aircraft interiors.

To keep the trains running on time, it makes sense to pay attention to the linear guides used throughout today’s railroad and mass transit cars.

Linear guides may not make an obvious contribution to the uptime of rolling stock, but they do play a crucial role in the operation of doors, access panels, seats and stowage systems. When linear guides fail, they can bring train to a halt as surely as a broken engine.

One way to avoid unnecessary failures is to pick reliable linear guides that are:

• Corrosion resistant. Exterior doors and access panels, in particular, need to withstand corrosion and must pass rigorous salt-spray tests before even being considered for use in railway applications.
• Contamination tolerant. Exposed or partially exposed linear guides have to tolerate a wide spectrum of human and environmental contaminants, including unusual ones such as hair.

Rollon supplies nine distinct linear guide styles to meet varying force, duty-cycle, integration and environmental requirements.

For a look at some strategies for fighting linear guide corrosion in railroad and mass transit applications, download our latest white paper.

Compact Rail

Machine builders are sometimes reluctant to switch from one type of linear guide to another because of perceived installation differences. The truth is, though, that these differences tend to be minor and may even bring some unexpected benefits. For a case in point, consider what’s involved when switching from a ball-rail linear guide to Rollon’s Compact Rail.

The Compact Rail can replace any similarly sized ball-rail guide within the size, load and accuracy constraints described in the last post. Think of Compact Rail as a potential replacement for ball-rail guides.

We say “potential” because there is a difference in mounting pitch. While ball-rail guides typically have 50- or 100-mm mounting pitch, Rollon’s roller-style Compact Rails have an 80-mm pitch. With just a few minutes of CAD work, this difference is easily accounted for during the design stage. If a machine frame has already been prepped for a 50- or 100-mm pitch, the process of switching to Compact Rail would take more effort because custom made interface plates and brackets would be required.

There is an aspect to Compact Rail installation, however, that offsets any minor difficulties related to mounting pitch. Because it can self-align, Compact Rail does not require truly parallel mounting surfaces. In fact, it can tolerate large frame misalignments without losing an iota of performance or suffering from additional wear.

How large? On rails 500 mm apart, for example, the Compact Rail system can tolerate rail height differences up to 20 mm. The system can also soak up misalignment in the horizontal axis–or when the rails toe in or toe out. The largest Compact Rail size can adjust to horizontal misalignments of nearly 4 mm over a 4080 rail length. The system can also accommodate angular misalignments up to +/- 2 degrees.

With ball-rail guides, the need to achieve parallelism during installation is one of the big cost drivers associated with the installation process. Mounting surfaces need to be ground or lapped carefully. And installers must go through the exacting–and expensive–process of shimming and bolting linear guides to the frame in a bid for near perfect parallelism.

In contrast to the exacting installation requirements for ball-rail guides, the Compact Rail typically adapts to the machine structure with a bit of screw tightening.This self-adjusting system does not require anything close to perfect mounting surfaces to work well. And that can be worth a lot more money than moving a few mounting holes.

Compact Rail

In all too many applications, engineers still pick linear guides based on familiarity rather than true accuracy, load and size requirements. And all too often, the most familiar linear guides are recirculating-ball styles with the longest lead times. Even a cursory needs analysis, however, will show that the majority of linear motion requirements can be easily met with advanced roller-style linear guides, such as Rollon’s Compact Rail.

Here are some specifics that can guide help you decide when an application needs can be satisfied with a lower-cost, readily available Compact Rail product–and when they can not:

• Accuracy. When the linear linear motion application requires positioning accuracy within 50 μ, expensive profiled rails really may be the only choice. Yet these applications are rare, as little as 10 to 15 percent of total linear motion applications. That leaves plenty of applications that could be running successfully on roller-style linear guides, which can easily meet accuracy requirements above the 50-μ mark. Rollon’s Compact Rail linear guides have the added benefit of being self-aligning in one or two axes, depending on the type of installation. Unlike ball rail guides, Compact Rail’s accuracy does not rely on an exacting installation of the rails on the machine frame.
• Load Capacity. If an application has a total load capacity in excess of 2,500 lb or moment loads in excess of 1800 Nm, a profiled rail system may be the only choice. These heavy duty applications are, again, rare. The vast majority of linear-motion axes involve relatively light loads under 1,000 lb. In these cases, the roller-style linear guides can again provide an immediately-available alternative to recirculating-ball linear guides.
• Size. Applications that need that need rail sizes about 65 x 65 mm are not a good fit for roller-style linear guides like the Compact Rail. These applications are, once again, rare with only 10 percent of applications requiring rails greater than 45 x 45 mm. In all other applications, Rollon’s Compact Rail can be a valid replacement for ball rail guides from a size standpoint.

Not all linear guides are created equal when it comes to delivery times. The highest-precision classes of linear guides usually have dramatically longer lead times, especially at lengths beyond 1240 mm.

For some axes of motion, the most precise guideways are indeed a necessity. But expensive precision products are routinely applied to axes that have less stringent motion requirements. Machine builders have in the past often specified one type of linear guide, usually a recirculating ball type, for use throughout the machine. This approach, while convenient, is no longer sensible or cost effective.

A classic example can be found on CNC machine tools. Motion axes related to cutting may well require the most precise, and expensive, linear guides on the market. The doors, tool-changer and other auxiliary axes are another story. These non-cutting axis may be better off with a roller-type guide that better matches the actual motion requirements–and offers low failure rates, superior alignment and reduced mounting surface preparation as side benefits.

Avoiding overkill on linear motion axes has obvious cost advantages because machine builders end up paying only for the precision they actually need. Less obvious are the supply benefits. Roller guides are almost always available for immediate delivery and can be ordered as needed. Using them where appropriate means there are fewer long-lead time linear guides that can cause delays.

Linear guide

Many industrial products experience supply crunches now and again. Linear guides are a prime example. Anyone who has ordered these crucial motion components over the years knows that delivery times periodically spiral out of control, increasing from weeks to many months.

The reasons for this cyclic availability are not hard to understand. Linear guides go into machines that are themselves subject to the boom-and-bust cycles of our economy. Making matters worse, linear guide suppliers rely on specialized supply chains that cannot react instantly to upswings in demand. After all, machining operations that can precision grind long pieces of steel to micron-level dimensional tolerances don’t spring up overnight.

Because it is so specialized, the supply chain for many linear guide products also tends to be concentrated and subject to disruption. The tragic earthquake, tsunami and nuclear accidents in Japan without a doubt play a role in the current sluggish state of linear guide deliveries. So does high demand from China and the rebound from the economic slowdown of 2009-2010.

And sluggish is truly an understatement. By our estimates, delivery times for linear guides now averages four to six months. Your actual delivery times may be less than that in some cases. If you are one of the best customers of the big linear guide suppliers, you may get top priority. The longest lead times of all are associated with high precision guides. You even may find yourself waiting months for the best recirculating ball-style guides if you don’t buy enough product to warrant the best delivery terms.

All of this may sound like Economics 101, but the cyclic availability of linear guides has very real cost consequences. Few machine builders can afford to delay machine deliveries for weeks or months while waiting for a linear guide that’s in short supply. Those that cannot or do not deliver on time may also face the costs of a damaged reputation, which can also add up over time.

It may seem like linear guide availability is beyond your control, especially if you’ve tried to place an order in recent months. Yet there is a way around the supply crunch: With the right technical information and design strategies, you can substitute different linear guides more freely in your machine designs and take advantage of a wider supply base.

In the posts that follow over the coming weeks, we’ll show the best ways to make use of Rollon’s linear guide products–all of which are available for immediate delivery.