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Often asked questions concerning self aligning bearings

Differences between  Self Lubricating PTFE Lined Linear Bearings
and Ceramic Coated Bearings

Some of the differences between liner ball bearings
and LM76 Minuteman
Linear Bearings


Break-Away Force For LM76 Self Lubricating Linear Bearing Slides:

To get a system in motion you must overcome the static weight of the system as it sits. It is important to
know this value when you are designing a drive system. Design for the breakaway force not the dynamic force.
Many of our competitors fudge the numbers by using the Dynamic Value in their sales literature.
For a Self Lubricating Linear Bearing Slide, we calculate the following:

Example: For a linear slide employing 2 rails and 4 Self Lubricating PTFE pillow block bearing
assemblies and supporting a horizontal, centered load of 250lbs.

Static Self Lubricating Linear Bearings  30% of System Load to get the system moving    
250lbs Load x .30 = 75 Lbs.              
  

Dynamic Self-Lubricating Linear  Bearings ( In Motion )  10% of system load                     
250lbs Load x .10 = 25
lbs.

Breakaway Force Requirements - Static

Self Lube PTFE  Dry Ceramic Coated
Lubricated
   30% of Load - Static   10% of Load - Static

PV ( Pressure Velocity )

PV is determined by 2 factors: Pressure ( PSI )
and Velocity (SFM)

Formula: P=W/(dxb)      
 W=Static Load on a    single bearing
 d= Bearing Inside  Diameter
 b= Bearing Length

Example 1:
225 pound load applied to a 1" inside diameter bearing that is 2.25" long
Note: Determining PSI: ID x Length
P=225/(1" x 2.25" ) or P = 100 PSI    Velocity = 50 Feet per minute
PV=100 (PSI)  x 50 FPM
Answer: PV=5000

LOAD CAPACITY EXAMPLES

Example 2 LOAD: LM76 Minuteman Self Lube Bearing vs. Linear ball 
bearing
. How does a 1" ID ( inside diameter ) LM76 Minuteman ( L1625-18SL ) 
compare against a comparable 1" ID Ball Bushing?

    A standard 1" linear ball bearing is rated @ 1050 pounds
    A direct drop-in replacement MINUTEMAN bearing is rated at: 11,250  pounds static load


   1" ID x 2.25" ( length ) = 2.25  x  *5000 PSI = 11,250 pounds

Example 3 LOAD: FDA/USDA Compliant Self Lube Bearing vs. Ball Bearing
Once again, we will use a standard 1" Ball Bearing with a Published Load Capacity
of 1050 pounds:

    Standard 1" ID Ball Bearing = 1050 Pounds
    A direct drop-in replacement 1" ID LM76 FDA/USDA Bearing with our white
    FDA compliant PTFE liner = 2250
    1" ID x 2.25" ( length ) = 2.25  x 1000 PSI =  2250 pounds static load      

EDGE LOADING:
Sleeve-type linear motion bearings, more so than ball bearings ( rolling element ),
require parallel shafting to be aligned as closely as possible to avoid edge loading.
Edge loading can cause higher-than-acceptable friction, binding and subsequent wear.
One answer for situations where shaft alignment is not true, is LM76's self aligning bearings.
These bearings have a slight radius on the OD ( outside diameter ) which allows the bearing
to rock in the housing and allows for .009 per linear linear inch misalignment. Another
option is a "full float" bearing which is suspended in the housing bore with o-rings.
Note: this remedy does change the shaft-to-bore alignment.

 

CANTILEVERED LOADING:
When you have a cantilevered load:

YOU MUST OBSERVE THE 2:1 RATIO RULE! Because moment loading can
result in edge loading of sleeve-type linear bearings, correct spacing between
bearings on a single shaft ( vertical or Z axes):                                        

      

and spacing between shafting on horizontal applications ( X,Y axes ):

must meet the 2:1 ratio rule. If this rule is violated, friction will increase, the system will
bind and the bearings will fail.

OPEN BEARING LOAD CAPACITY

NOTE:  Inverting ( hanging upside down ) open style sleeve bearings is not an optimal
                design configuration. System load is forced into the bearing slot - the weakest
                part of the bearing. Depending on load and possible moments, point loading on
                the edges of the slot can result in hot spots, liner cold flow and excessive wear.
                Please call LM76 if you are employing this configuration in your design
.
                1-800-513-3163

 

Straight Downward
Compressive
Side Mounted Inverted
(upside down)
100% 70% 30%

   
 

Coefficient of Friction Graph

The graph illustrates the coefficient of friction values for:

  • Ball Bearings
  • LM76 Ceramic Coated ( lubricated )
  • Minuteman Self Lubricating Linear Motion Bearings

Note 1: That the LM76 Ceramic Coated bearing offers a very low coefficient of friction as opposed to
the Self Lube Bearings. If you can lubricate, the LM76 Ceramic Coated bearing will ensure
low friction, long life, low cost and unlimited velocity.

Note 2: all LM76 self lubricating linear bearings cannot exceed 400SFM.

1-800-513-3163    413-525-4166   Fax: 413-525-3735