INTRODUCTION TO LINEAR MOTION GUIDES ;
Linear
Motion Guides are those precision machine elements which are designed with friction
less, noise less, vibration free and other working characteristics to permit accurate
linear relative motion between two machine members. LM Guides are available in
two parts, one is LM bearing or carriage and another is LM shaft or rail over
which carriage moves. These guides are made in various shapes, sizes and
materials depending upon load, length of travel, speed of travel, space availability
and working conditions. Linear Motion Guides find applications in almost every
industry from General Purpose Machine Tools to Special Purpose Machine, from Medical Equipment to Aerospace and from Transport to Construction. All the LM
guides can be classified broadly into three categories depending upon the type
of contact in the guide elements during motion i.e., (i) Sliding Contact, (ii) Rolling
Contact and (iii) Magnetic field.
Classification of Linear Motion Guides ;
Classification based on technology incorporated to achieve motion or the type of contact in LM Guide elements ;
1. Sliding Contact Linear Motion Guides ;
In these
guides, motion takes place because of sliding action between guide elements
i.e., LM block and LM guide rails or rods. These are also known as Plain LM
Guides. Sliding Contact LM Guides can be further classified in to two
sub-categories, depending upon thickness of lubrication film in between sliding
surfaces, as given below ;
a) Thin Lubrication Film Sliding Contact LM Guides
; A partial contact remains in between guide elements as a thin layer of
lubricant sweeps in between them during motion. Low friction coefficient materials
are used in construction of this category of Guides. Despite of some advantages
like simple in design, low cost, etc., these guides, however, least preferred
because of certain disadvantages like high
friction, high heat generation, less efficiency, low life, low load carrying
capacity, high maintenance.
b) Hydrostatic LM Guides ; In these types of
bearings a thick layer of pressurized fluid holds the carriage and there is no direct
contact between the moving elements. To overcome disadvantages of high
friction, high heat, etc. pressurized fluid is supplied in between moving
elements forcibly from external source, which makes these guides costly because
it requires additional unit to supply fluid, not suitable for applications in
clean-rooms as there are always chances of leakage of pressurized fluid, etc.
Examples of Sliding Contact
Linear Bearings are i) Spline Shaft & Spline Nut, ii) Slide Rails, iii)
Lead Screws, etc.
2. Rolling Contact Linear Motion Guides ;
These LM
guides consists of balls or cylindrical rollers or wheels of various shapes or profiles which enable smooth and very low frictional motion at fairly high speeds in
between carriages and their guides or rails. Despite of being comparatively costly these
LM Guides are most preferred because of certain advantages like trouble free
operations even at high speeds, compact design, low maintenance, high load
carrying capacity, high efficiency, etc.
Examples of Rolling Contact
Linear Motion Guides are ;
i) LM Guide Blocks and Rails, and ii) Ball Screws,
in which hardened spherical steel balls keep on circulating in a continuous
end-less grooves of carriages while motion. These spherical balls support the
carriage over rail or shaft which is firmly fixed to one of the machine members as well
as facilitate smooth movement of loads,
iii) LM
Roller Guide Blocks have cylindrical rollers, which facilitate relative
movement in between carriage with greater load and rail, and
iv) Guide
Wheels and Rails, in which wheels with a profile usually Circular groove (semi-circle) or V
groove in its outer face along circumference is provided. Also a protrusion of
similar profile like Guide Wheel’s groove profile in rail is provided which fits into Wheels and guides
them. These are preferred where movements of comparatively greater loads at
higher velocities are required.
a) High
Positional Accuracy,
b) High
Efficiency,
c) High
Speed Motion of loads is possible,
d) High
Loads movement is possible,
e) Low
Noise,
f) Less
Maintenance,
g) Fairly
good Life.
3. Magnetic Field Linear Motion Guides ;
These guides preferably use electromagnets to
provide relative linear movements in between two machine members. Their
application is limited to only clean rooms in medical and other such similar clean and tidy places as no
contact between parts having relative motion eliminates use of any kind of
lubrication. Disadvantages like high cost, space constraints, etc. makes them less popular.
Types of Linear Motion Guides
Different types
of LM Guides, which falls in any of the above mentioned categories, available
in markets have been explained below ;
1. Linear Motion Block & Rail ;
These blocks are highly precise and compact in
design, in which 4 rows of steel balls keep on running in continuous end-less
paths. The rolling balls remain in firm touch with 4 corresponding raceways on
rail and supports block on rail and enable linear movement along the rail. The
re-circulation of balls also permits the motion of load along desired length by
introducing additional lengths of rails. These guides come in category of
rolling contact LM guides and are available with either spherical hardened
steel balls or hardened cylindrical rollers, which also re-circulate or guide
wheels to achieve rolling action. These are most commonly used guides because
of certain advantages like high accuracy, high efficiency, availability, high
speed motion, etc.,
2. Linear Motion Ball Bearings & Shafts ;
These are also a type of LM guides which
permits linear relative motion. LM Ball Bearings are made in ring shape which
carries continuous end-less raceways along its length to guide hardened
spherical steel balls during motion. And as the bearings are in ring shape,
they deliver motion through cylindrical shafts.
These bearings are available in various shaft sizes and types like
closed-type and open-type, with or without flanges, Pillow blocks, etc.
Open-type bearings are made cut open along its length to guide shafts
mounting on fulcrums. These guides also come in Rolling Contact LM Guides.
3. Linear Motion Stroke ;
These are also made in ring shape
like LM bearings, except it has a drilled ball-cage in which holes are arranged
spirally along the surface of cage and so the balls are and steel balls do not
re-circulate unlike LM bearings. After inserting balls in holes of cage, holes
are mechanically deformed through suitable process usually caulking, which
narrows holes and cage retains balls and they do not fall. LM Strokes are also
rolling contact type and they are suitable for rotational and reciprocating motions.
These are used for small distance and high speed movements.
4. Linear Motion Spline Nut & Shaft ;
A Spline shaft is a
round shaft in which a series of continuous equally spaced grooves are cut
along the length of the shaft. Spline shafts, therefore, has teeth along its
periphery and gives gear like structure when looking from cross-section
area. Thus, spline nut has similar teeth
at its internal surfaces, which fits into corresponding grooves on shaft and
hence provide stable sliding motion along shaft length. These are Sliding
Contact type LM Guides. These are less efficient, they have high friction, low
positioning accuracy. Advantages
are ; Low Cost, Good Life, Less
Maintenance, etc.
5. Lead Screw ;
These screws normally have square
threads instead of V threads as they are more efficient. Lead Screws are used
to convert rotational motion of screw into linear motion of Nut. Backlash, low
efficiency, high wear are the disadvantages of these screws.
6. Ball Screw ;
These guides like Lead Screws
convert rotational motion of screw into linear motion of Nut. Unlike lead
screws, these are highly efficient because rolling motion of steel balls in
between grooves cut in the nut and screw shaft. Steel balls re-circulate in the
groove. Wear rate in these guides are much less.
Other LM
Guides available in market are ; Ball Spline, Slide Rail, Guide Wheels and some
LM Tables with or without drive.
Applications of Linear Motion Guides ;
Some common applications are ;
LM Block |
- Machine Tools like Horizontal Machining Centres (HMC), Vertical Machining Centres (VMC), CNC machine tools, etc.,
- Industrial Robots,
- Medical Equipments,
- Electronic Equipments,
- Special Purpose Machineries.
Selection of Linear Motion Guides ;
- Load to be moved,
- Moving Speed of Load,
- Space Availability,
- Accuracy,
- Length of Movement,
- Efficiency,
- Working Conditions, &
- Inter-changeability & Easy Availability
A thought from personal experience ; An LM Block was broken
during my service to a Diaper Manufacturing Company. The constraint was that
the machine manufacturer was Chinese and spare was not available with us or
anywhere in Indian market. It was also not possible to get it delivered from
China as it would have taken long time. So, I checked model number of LM Block
engraved on it and searched it over internet. I found company's catalog and
after comparing all the specifications of LM block with Indian manufacturer, I
got part number of local brand Block. I ordered block and in that case rail
also had to purchase. Just because of that I had put a point of Easy
Availability in Points of Selection.
Guides also need to be
protected from dust for which manufacturers provide some accessories like
bellows to cover rail and open parts of carriage.
Few other posts seeking your attention are ;
1. Hooke's Law
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