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When design engineers evaluate linear bearings, they
always ask about perormance attributes such as speed,load capacity and liecycle. Then they want to know the
price. Its rare, however, that they ask about the bearings
sensitivity to misalignment. And thats a big mistake,
because misalignment represents one o the leading
causes o premature linear bearing wear and ailure.
Linear bearings that should last or years based on
expected lie calculations can quit ater a just ew months
i they are not aligned to the geometric tolerances they
require to run smoothly. Usually, alignment problems begin
with the design and preparation o the machine rame
itsel. It may not be at, straight or parallel enough or a
bearing to mounted properly. For example, mounting
suraces may have one or more high spots that will read
through to the installed bearing rails. Or the rame design
may make it difcult to mount bearing rails parallel to one
another in the horizontal axis, vertical axis or both.
Whatever the type o misalignment, the result is uneven
loading o the bearings rolling elements and raceway
suracesincluding excessive point loads. These uneven
loads typically cause wear in the orm o pitting. Just as a
pothole in the road starts small and then grows as more
cars drive over it, pits on the rolling element and raceway
suraces grow with each pass o the carriage. At some
point, even beore it ails catastrophically, the pitting can
cause the bearing to become noisy and sluggish.
By shortening the working lie o linear bearings,
misalignment can be a signifcant cost driver or both the
machine builder and owner. Machine builders suer rom
higher warranty costs when bearings ail prematurelynot
to mention the less tangible cost o a damaged reputation
or quality. Machine owners, meanwhile, have to contend
not just with the cost o buying and installing new bearings
but also any downtime costs.
MANAGING MISALIGNMENT IN
LINEAR MOTION SYSTEMSOf all the factors that contribute to the premature failure of linear bearings,
misalignment ranks near the top of the list. Heres how to cope without breaking
the bank.
Rollons Compact Rail system is inherently tolerant o misalignment,
thanks to a rail geometry that can absorb alignment errors in one
or two axes.
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Rather than incur these costs once the machine has
gone into service, its ar better to deal with misalignment
upront. In general, there are two ways to do so. The hard
way involves design and manuacturing procedures that
attempt to eliminate misalignment altogether. The easy
way accepts misalignment as a act o lie and employslinear bearings that inherently have a wide alignment
tolerance. Both o these strategies have their place, but
they also have drastically dierent cost implications.
ThE hard Way
Linear guides that use recirculating balls are well-established
in high precision motion applications or good reason.
When properly installed and maintained, these linear guides
are designed to meet the stringent positioning accuracy
requirements o machine tools and other precision industrial
machines. In act, the best o these guides can be ound on
motion axes that oer repeatability at the micron scale.
But this kind o precision doesnt come cheap. It requires
machine builders take expensive steps to create nearly
perect mounting suraces or linear guides. Some ultra-
high precision guides, or example, call or mounting
suraces that are straight, at and parallel within a ew ten-
thousandths o an inch.
This process o eliminating misalignment starts when the
machine is still on the drawing board. To accommodateultra-high precision guides, design engineers will oten
have to speciy pricey rame materials and abrication
methods that will acilitate the creation o at, straight,
parallel mounting suraces. Typically, the required geometric
tolerances will call or precision grinding and lapping
operations whose cost rises exponentially with the length o
the linear axis.
Whats more, misalignment can also result rom deections
o the mounting surace when loaded. So ultra-high
precision guides may also need engineers to bee up partso the machine rame in order to provide linear guides
with a mounting surace that is rigid enough to prevent
deection.
Steps to combat misalignment also take place on the
assembly oor. Assemblers oten have to pull linear guides
into alignment inch by inch, using custom fxtures, fnicky
bolt adjustments and shims. This process is nothing new
Figue 1. axil Pllelim Eo
By combining Rollons T and U Rails, machine builders can stop
worrying about rail parallelism in the horizontal plane. The shape
o the U Rail eatures a at raceway that gives the roller enough
lateral reedom to absorb large (see table) deviations rom
parallel.
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or many machine builders, but it is time-consuming and
expensive. And like machining, costs rise with the length o
the axis.
ThE Easy Way
The other broad strategy or dealing with misalignment is to
live with it by choosing linear bearings that can sel-align. In
contrast to recirculating ball systems, another type o linear
bearing eature large rolling elements, rail profles that give
the rolling elements some wiggle room and a simple preload
adjustment that enable equal loading o all rolling elements
The Compact Rail System rom Rollon Corp. is a prime
example o this misalignment-tolerant design. Its rollers have
enough rotational and lateral reedom within the raceways
to oset even large misalignments in all axes. Whereas ultra-
high precision guides measure acceptable misalignment
in arc minutes and microns, the Compact Rail system
measures it in degrees and mm.
For example, Compact Rail rollers can rotate up to 2
degrees relative to the rail without aecting unctionality or
increasing wear. This rotational reedom allows the system
to accommodate a 20 mm dierence in rail height when
the distance between rails is 500 mm. Likewise, the rollers
ability to translate laterally allows it to adjust to parallelism
problems in the horizontal axisthat is, when rails toe in or
toe out. The largest Compact Rail size, or example, can
adjust to displacements up to 3.9 mm over a 4080 mm rail
length Finally, because the elements are so large and can
move within the raceway, they can also adjust to localized
variations caused by high spots in mounting suraces or by a
less than exacting assembly process.
For machine builders, the benefts o a sel-aligning system
come down to design reedom and cost reduction. When
the condition o the bearing mounting surace becomes
less critical, it is easier to design all or part o a machine
rame using lower cost materials and abrication methods.
Compact Rail, or instance, has been directly mounted
Figue 2. alignment Eo In Two Plne
K tle U tle
Rollons K and U Rails work together to absorb alignment errors
in two planes. The K Rail geometry gives the roller a rota tional
reedom that osets dierences in rail height, while the U Rails at
raceways allow la teral reedom to oset parallelism errors in the
horizontal plane. The system will tolerate signifcant misalignment
(see graph) without an eect on wear or lon gevity.
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to sheet metal, a surace that would be too compliant or
conventional linear guides. Gone too are the expensive
grinding o mounting suraces and the ussy assembly
methods.
WhIch Way?
When comparing the two approaches to misalignment
eliminating it or adjusting to itkeep in mind that both
have their place. Some linear axes truly do need a rigid
bearing with the best possible precision. In these cases,
there is little choice but to spend the money on machine
rame upgrades, precision grinding and careul assembly.
Others axes, ones with slightly lower precision and accuracy
requirements, will be better served by a more compliant
guide that can align itsel to imperect mounting suraces.
Costs in these cases will be lower.
What machine builders sometimes ail to realize is that these
two approaches are complementary. Many machines will
have linear axes with dierent accuracy and precision
requirements. Consider machine tools, or example. The
spindle may need the most accurate linear motion systemmoney can buy, while the tool changer and door do not. All
too oten, however, the same type o precision bearing that
is required or one part o the machine will, by deault, be
used throughout all or most o the machine, driving up costs
unnecessarily.
A better way to go is to pick the best linear bearing or
each axis individually. And whenever a sel-aligning style will
meet the accuracy requirements or a given axis, it will save
money.
The Compact Rail system oers three
dierent rail profles, which can be
combined to compensate or dierent
types o misalignment.
T U K
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To get the longest working lie out o a linear bearing, keep it
clean and well lubricated. This common-sense advice may
sound easy enough to ollow. Yet in the real world o round-
the-clock, high-cycle manuacturing operations, bearings
do get dirty and dry.
And when either o these conditions happens, linear
bearings will wear prematurely. In the worst-case scenarios,
contamination and inadequate lubrication can create
metal-on-metal contact between the bearings rolling
elements and raceway. This can cause excessive wear in
the orm o denting, pitting or galling results.
This warning about contaminants and the importance o
lubrication will not come as news to anyone who has
designed or worked around industrial machines. When using
linear guides on medical, ood, packaging, semiconductor
or other sensitive equipment, machine builders oten takeextraordinary measures to keep the contaminants out and
the oil in. They may add expensive bellows to cover the
guides, or they may opt or a pricey automatic greasing
system.
Yet in their zeal to keep linear motion systems running
smoothly, machine builders can overlook less expensive
design solutions to contamination and lubrication issues.
cOMBaTinG cOnTaMinaTiOn
Contamination comes in many orms, some more
aggressive than others. Metal chips rom machining
operations, or example, qualiy as one o the biggest
oenders rom a wear perspective. Silicon dust produced
during semiconductor manuacturing can also be tough
on linear bearing suraces. Modern manuacturing
processes can throw o a long list o other abrasive, wear-
inducing contaminants.
Less aggressive contaminants can pose problems too.
Even sot contaminantssuch as those ound in ood
processingcan gum up linear bearings. This kind
o debris is not necessarily a wear issue, but it can keep
the linear motion systems rom unctioning smoothly.
Consequently this can have a negative impact on
positioning accuracy and product quality. I a linear
bearing gets gummed up to the point it stops working, then
there are also potential maintenance and downtime costs.
Keep in mind, too, that contamination is a two-way street. In
addition to worrying about contamination rom the product
REDUCING LINEAR BEARING WEARContamination and poor lubrication practices can shorten the life of linear bearings.
Here is a look at how to avoid both problems.
Unlike recirculating ball linear guides, Rollons Compact Rail
system utilizes large rolling elements that can pass over any debris
that all into the rail.
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interering with the linear motion system, machine builders
also worry about stray lubrication or particulate rom the
linear bearing contaminating the product. This type o
machine-to-product contamination is a cause or concern
in contamination-sensitive industries such as medical,
semiconductor and electronics.
To combat contamination, machine builders will oten
supplement the linear bearings built-in seals with bellows or
other types o covers. Though these can substantially add
cost to a machine and add to the maintenance burden,
they do have their place. Some clean-room environments,
or example, may require some physical barrier between
the product and the machine elements. And insidemachining centers, it is crucial to physically protect any
motion systems rom metal chips.
But there are many less severe contamination scenarios.
And in these cases, machine builders should consider
bearing styles that are less aected by contamination and
less prone to generate any o their own. Bearings with large,
sealed rolling elements all into this category.
With conventional linear guides, the tiny recirculating balls
in a raceway have very little clearance. So even a relatively
small pieces o debris can interere with the balls. Bearings
based on larger diameter rollers, by contrast, can roll right
over even relatively large contaminants. Think o it as the
dierence between a skateboard and monster truck hitting
a speed bump in the road.
Rollons Compact Rail system, or example, is built around
rollers that are an inch in diameter. These linear bearings
can roll over all kinds o contaminants that can stop smaller
rolling elements dead in their tracksincluding metal chips,
plastic particulate, paper dust and more.
Large rollers are also more damage tolerant than smaller
rolling elements. Even i a contaminant does happen to ma
the roller or the rail suraces, the large rolling element can
usually keep on running.
WEll-OilEd MachinEs
Lubrication problems are oten described in terms o not
enough grease or oil. And sometimes that is the case as
lubrication leaches out or is squeezed out o linear bearings
leaving them susceptible to metal-on-metal wear.
There is a ip side to the lubrication issue too. Maintenance
workers, with grease guns in hand, oten over-lubricate
linear bearings, potentially blowing out seals and
introducing oil into environment.
And keep in mind that lubrication also qualifes as a
contaminant. So over-lubed bearings can also worsen
contamination problem or orce machine builders to opt or
bellows or physical covers.
Rollons Compact Rail ball bearings are permanently lubricated
and sealed within the rollers, which isolates these small rolling
elements rom contaminants.
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Getting just the right amount o lube into a linear bearing
at the right interval can be tricky because it depends on
application-specifc actors such as the type o bearing
and the duty cycle. The applications sensitivity to oil
contamination comes into play toowith sensitive industries
demanding lighter lubrication schedules.
There are ways to make sure linear bearings stay properly
lubricated throughout with little intervention on the part o
maintenance workers. On the expensive end o the system
are expensive automatic greasing systems. These systems
are a legacy o the linear way systems used in the machine
tool industry. The auto-greaser was crucial or maintaining
the flm o oil that separate a linear ways bearing suraces.
Some o these expensive auto-lubrication systems have
made their way into applications that employ more modern
linear guides, and they can be a valid way to get the lube
levels right while reducing the maintenance burden. But
aside rom their cost, auto-greasing systems can introduce
unacceptable levels o contaminant into the actory
environmentwhich, again, will create problems is an issue
in clean-room and other sensitive manuacturing operations.
A lower-cost, cleaner alternative to automated lubrication
exists in the orm o sel-lubricating wipers that integrate into
the linear bearings carriage. Rollon, or example, has an
optional sel-lubricating wipers on its Compact Rail system.
These provide lubrication or 2 million cycles beore they
need to be rereshed, and they cost a raction o what PLC-
controlled auto-greasing systems cost.
The very idea automatic lubrication is the only way to
achieve optimal lubrication is somewhat outdated. Some
linear guides, such as those based on recirculating balls, do
require regular lubrication in all applications. But bearings
based on sealed rolling elements may not need much, or
any, external lubrication at all in many applications.
With Compact Rail, the roller acts as a true sealed bearing.
Its ball bearings, raceway, and their lubricant are contained
inside the outside housing o the roller. The need to add
lubricant between the roller and the profled rail thus
becomes less importantor even unnecessary in light
duty applications. This capability makes a big dierence in
contamination-sensitive applications industries that would
normally resort to expensive bellows or covers to keep stray
oil o their shop oors, out o their air and o their products.
The Compact Rail slider incorporates an integrated
wiper (see inset) that automatically satisfes the
systems minimal lubrication requirements.
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Look through the specications o any linear bearing, and
youll come across a maximum speed. Its an important
spec, but not not nearly as important as many engineers
think it is.
Speed does play a role in bearing lie. Consistently
moving loads above a bearings speed limit can result in
premature wear. It is also possible, usually though a control
error, to drive a bearing so ast that it will ail. For example,
recirculating ball linear guides can experience end cap
ailure when driven at high speeds or accelerations. And
roller bearings can reeze up at extremely high speeds.
Yet these over-speed conditions are rare in well-designed
motion system.
Whats more, bearing speed is rarely the limiting
perormance actor in todays motion applications.
Standard linear bearings can handle speeds o 3 m/s
without any diculty, while high-perormance models
can reach 5 m/s. Some bearings are even aster, with top
speeds approaching 9 m/s. The majority o automation
applications, meanwhile, run at top speeds well under 5m/s.
So i todays bearings have speed to spare and over-speed
conditions are rare, what good are speed specications?
The answer is that speed doesnt tell us all that much as
stand-alone value. Only when considered in the context
o acceleration and the overall motion prole does speed
become important.
UNDERSTANDING LINEAR BEARING
SPEED & ACCELERATIONEngineers focused on fast, light automation systems may pay too much attention to
speed specs when acceleration is what really counts.
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AccElErAtion considErAtions
There is a connection between speed and acceleration
beyond their mathematical relationship. Bearings that
are engineered to withstand high speeds also tend to be
robust enough to withstand high acceleration and vibration
orces.
These are the orces that make or break most real-world
motion applications. Acceleration and its rate o change
(or jerk) are the chie determinants o a motion systems
stability and ability to hit position targets.
When control engineers do overtax a linear bearing, excess
velocity is rarely the culprit. More typically, the engineer has
pushed the system past its ability to handle acceleration,
deceleration or vibration orces. In mild cases, this condition
can maniest itsel as skidding, which can increase wear
and shorten bearing lie. In extreme cases, excessive
acceleration may result in catastrophic bearing ailures.
Working With thE MEchAnics
Control engineers can obviously limit these orces when
they program a motion prole, but only i the prole
realistically relates to the bearings mechanical limitations.
All too oten it does not. Motion proles are routinely
created around speed targets without reerence to the
undamental limitation o the mechanical components.
When this disconnect between the motion prole and the
mechanical components happens, no amount o drive
tuning will help.
And keep in mind that not all the remedies or acceleration
diculties involve the motion programming. Preloading the
bearing, which is purely a mechanical process, can help
too.
Some amount o preloading is oten desirable because it
removes excess clearance between the rolling elements
and the rail suraces, eliminating uneven contact conditions
Trianglular Motion Prole
Velocity
Accelerate(Time in seconds)
Accelerate(Time in seconds)
Decelerate(Time in seconds)
Decelerate(Time in seconds)
Constant Velocity(Time in seconds)
Velocity
Vmax
ta
ta
td
td
tc
Vmax
Trapezoidal Motion Prole
Velocity
1A 1B
Time
Acceleration
Velocity
Acceleration
Phase
I
Phase
II
Phase
III
Phase
IV
Phase
V
Phase
VI
Phase
VII
Time
Time
Time
Phase
I
Phase
II
Phase
III
Phase
IV
Phase
V
Phase
VI
Phase
VII
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Lkm
C
P=100 f
h
3
fc
( )Where:Lkmis the theoretical life in kilometerCis the dynamic load rating in NewtonsPis the equivalent external load in Newtons
fcis the contact factor
fiis the service factor
fhis the stroke factor
Dynamic loads obviously
play a role in how long linear guides
will last. But so does their operating speed.
In the lifecycle calculation (below), the service
factor term (fi) captures the influence of operating
speed, shock and vibration loads, reverse frequency and
the cleanliness of the working environment. Speed, in
particular, has a strong influence on service factor.
Low-speed applications (less than 1 m/s)
have a service factor of 1 to 1.5
Medium-speed applications (1 and 2.5 m/s)
have a service factor from 1.5 to 2
High speed-applications, those above 2.5 m/s,
have a service factor ranging from 2 to 3.5
All else being equal, a bearing in a high speed
application could conceivably have a
lifespan that is 50 to 70% shorter than the
same bearing in a low-speed
application.
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and minimizing wear. As acceleration rises and skidding
becomes a risk, keeping the rolling elements in contact
with the rail at all times becomes even more important.
Increasing the preload also makes the system stier, which
helps improve positioning accuracy when acceleration, jerk
and vibration orces are high.
In general, high acceleration applications demand a
higher preload. But there are a couple o trade-os in that
preloading reduces the bearings load carrying capacity
and increases wear. These trade-os can be kept to a
minimum by experimenting to optimize the preload levels.
There is usually a level at which the needs o an aggressive
motion prole can be met without sacricing much load
carrying capacity or lie expectancy.
Experimenting with dierent preload levels will require some
physical adjustments to the bearings. And not all bearing
systems make this task all that easy. Some systems have their
preloads determined by their actory set-up. The preload
on recirculating ball guides, or instance, relates to the size
o the ball bearings in relation to raceways. Changing the
preload on these systems typically requires changes to the
slider, the rails or both.
Other bearing styles allow simple preload adjustments in
the eld. Rollons Compact Rail system can be adjusted
rom outside the rail by turning screws that compress the rail
fanges. Aside rom allowing adjustments without removing
the slider rom the rail, this system allows localized ne-tuning
Preloads can be set higher in areas where high dynamic
loads occur--or example, where the carriage reverses
direction. Minimizing the amount o the rail subjected to
higher preloads reduces the wear eect accordingly.
It may always be tempting to look at a linear bearings
maximum speed to see i it will do the trick in a ast motion
system. But ocusing solely on speed without regard or the
underlying mechanics o the linear bearing system will only
slow you down.
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How do you know when a linear bearing has reached its
end o travel? When the carriage hits the wall. At least, thatis how the old engineering joke goes. But in reality, crashes
are no laughing matter.
Also known euphemistically as hard stops, crashes occur
when an out-o-control pillow block slams into the bearings
end stop or some other intermediate target. They most
oten occur when a linear axis is started up or the rst
time. Even a single crash on that very rst stroke can ruin a
bearing, triggering expensive repair or replacement costs.
The reason why bearings crash on start up mostly comes
down to human actors. A control engineer can do a
great job calculating the perect motion prole or a given
application, only to overlook some installation details during
start up. Small errors like entering an incorrect motion
parameter or ailing to connect a limit switch are common
and can be catastrophic or the linear bearing.
Fortunately, it is not that dicult to minimize crashes and
give your bearings a longer lease on lie without spending
a lot o money. Heres how:
gEt on thE sAME PAgE
All too oten there is little communication between the
engineer who designs a mechanical system and the
engineer who ultimately runs it. This lack o communication
can make crashes more likely because it increases the
likelihood that the linear axis may be run with motion prole
it was not designed to handle.
For example, a linear axis may have adequate space or
over-travel based on the design inputsloads, speeds,
accelerations and inertia mismatch. Yet i it runs under
dierent conditions, that over-travel space can shrink or
disappear.
To avoid this kind o problem, it is important or both design
and control engineers to have a sense o whether the
design values leave enough fexibility to accommodate
changes in real-world operating parameters. I not, a less
aggressive motion prole may be the only way to avoid
crashes.
CRASH-FREE LINEAR MOTIONOne of the greatest threats to the long life of a linear motion system usually
occurs on its very first stroke.
Rollons Compact Rail eatures a robust design that withstands all
but the most severe crashes.
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Avoid ovEr-EnginEEring
Other than busted bearings, one o the negative
consequences o crashes is over-engineering. Instead o
trying to avoid crashes, the engineer will accept them as
inevitable and try to design systems that will survive them.
To do so, engineers may end up speciying bumpers or gas
shocks. Depending on the type o shock absorber, these
can cost as little as a ew bucks and as much as $100 each.
But install them on multiple axes, and those costs can add
upeven beore you actor in the labor and spare part
costs.
As popular as they are, bumpers and other shock absorbers
are a bit like training wheels on a bike. Once someone
knows how to ride, o they go. Likewise, linear motion
systems that are properly designed and controlled can run
saely without the expense o additional protection.
Another crash protection strategy involves beeng up
linear motion components. Engineers will sometimes size
the bearings to survive crashes rather than to meet the
applications load, speed and acceleration requirements.
This strategy may not add much cost on a single axis,
but upsizing the linear motion components on an entire
machine can get expensive. All the more so i all those
upsized bearings trigger a shit to larger motors, gearboxes
and raming elements. When upsized bearings lead to this
kind o pervasive over-engineering, it would not be unusual
to see 30 percent increase in total machine cost.
FAvor CrAsh-worthy BEAring dEsigns
While most crashes do occur on start up, it is true that
they occasionally occur well ater a machine has been
commissioned. Sometimes a loss o power can trigger a
crash, or someone might inadvertently change a control
parameter. And truth be told, some machines are not run
all that careully.
For these reasons, it can make sense or engineers to designwith some crash-worthiness in mind. But what is the best
way to do that? Engineers can choose to go with shock
absorbers and over-engineered components, accepting
those costs as the price o crash protection.
Or they can avor linear bearings that inherently have
crash-worthy construction. Not all bearings are created
equal when it comes to surviving a crash. Recirculating
ball systems whose pillow blocks have plastic end caps are
notoriously susceptible to crash damagebecause the
plastic tends to shatter. Even a single crash with this type o
pillow block, and you may nd its ball bearings all over the
foor.
Rail Length
Motion
Prole
Crash
Rail Length
No Crash
Crashes most commonly occur on start-up due
to an incorrect motion profle.
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Bearings based on larger roller elements lack this particular
Achilles heel. Their ball bearings and their raceway are
contained within the roller element, not within a plastic
cap. Rollons Compact Rail is a prime example o this type
o bearing, and it will run just ne with a shattered endcap. And i it ever does experience any crash damage, its
interchangeable rollers and pillow blocks allow it to be easily
repaired without changing the rails.
Given the choice between crash avoidance and crash
protection, engineers can save the most money by avoiding
crashes in the rst place. And it is true that careully
designed motion systems with room or unexpected over-
travel are unlikely to crash.
Yet in the real world, it pays to be prepared or the
occasional hard stop. With the availability o crash resistantroller bearings, crash protection does not necessarily require
the expense o adding shock absorbers and over-sized
components.
The end caps on Rollons Compact Rail (top) are cosmeticand, even i broken in a crash, will not interere with the operation
o the slider. End caps on recirculating ball linear bearings
(bottom) house the crucial rolling elements and are a potential
point o ailure i shattered in a crash.