| INCh AND METRIC BALL SCREW ASSEMBLY
GLOSSARY AND TECHNICAL DATA |
| THREAD FORM TERMS |
LOAD DEFINITIONS |
DESIGN CONSIDERATIONS |
BALL SCREW SELECTION |
Introduction
Bearing Ball Circuit
Return Guide
Load Carrying Balls
Land Diameter
Ball Circle Diameter
Root Diameter
Pitch
Lead
Screw Starts
Lead Accuracy
Matched Lead
Straightness
Life
Efficiency
Backdriving
Backlash
Selective Fit |
Static
Load
Dynamic Load
Tension Load
Compression Load
Thrust Load
Overturning Load
Side Load
PreLoad |
Mounting
and Pinning
of Ball Nut Flange
Standard Flange Orientation
Proper Ball Nut Orientation
Transfering Ball Nuts from Shipping Arbor
Installing SEL, SAR, AND SAG Ball Nuts
Lubrication
Driving Torque
Backdriving Torque
Temperature
End Machining
EZZE-MOUNT
Optional Surface Coatings
Wiper Kits
Boots and Bellows |
Introduction
Load
Speed
Length
Life Expectancy
End Fixity
Critical Speed
Column Strength |
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| THREAD
FORM TERMS |
INTRODUCTION
Ball screws offer an efficient means of converting rotary motion
to linear motion. A ball screw is an improvement over an acme screw
just as an antifriction ball bearing is an improvement over a plain
bushing.
Ball screw assemblies have a number
of bearing balls that transfer the load between the nut and screw.
The thread form in which the bearing balls ride is an ogival shape
formed from two arcs of the same radius with offset centers. This
form is also referred to as a gothic arch. (SEE FIG. 1) |
| FIG.
1 |
 |
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BEARING
BALL CIRCUIT
The closed path that the bearing balls follow through the ball nut.
Ball nuts may have one or more circuits.
RETURN
GUIDE
When bearing balls circulate in a ball nut, a ball enters the ball
path between the nut and screw carrying the load one or more turns
around the screw. The bearing ball is then picked up and returned
to the beginning of the circuit through the return guide.
LOAD
CARRYING BALLS
The bearing balls in contact with ball nut and ball screw sharing
the load.
LAND
DIAMETER
The outside diameter of the screw. This diameter is less than the
ball circle diameter.
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BALL
CIRCLE DIAMETER
The diameter of the circle generated by the center of the bearing
balls when in contact with the screw and nut.
ROOT
DIAMETER
The diameter of the screw measured at the bottom of the thread.
This is the diameter used for column strength, critical speed calculations
and end machining considerations.
PITCH
The axial distance between threads. Pitch is equal to the lead in
a single start screw.
LEAD
The axial distance the nut advances in one revolution of the screw.
The lead is equal to the pitch times the number of starts.
LEAD
= PITCH x STARTS
SCREW STARTS
The number of independent threads on the screw shaft; typically
one, two or four. (SEE FIG. 2)
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| FIG.
2 |
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LEAD
ACCURACY
Lead accuracy is the difference between the actual distance traveled
versus the theoretical distance traveled based on lead. For example:
A screw with a .5 inch lead and ±0.001 inch per foot lead
accuracy rotated 24 times theoretically moves the nut 12 inches.
24 Revolutions X .500 inches per revolution
= 12.000 inches of travel with a Lead accuracy of .001 per foot,
actual travel could be from 11.999 to 12.001 inches.
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PowerTrac™ SRT Ball Screws
will not deviate from nominal lead by more than ±0.004 inch/foot
on screws through 2 1/2" diameter and ±0.008 inch/foot
on screws 3" and over.
PowerTrac™ XPR Ball Screws
will not deviate from nominal lead by more than ±0.001 inch/foot.
PowerTrac™ SGT Ball Screws
will not deviate from nominal lead by more than ±0.0005 inch/foot.
MATCHED
LEAD
When multiple screws are used to move a load with precise synchronicity,
screws of similar lead accuracy can be factory selected and supplied
as sets. Consult factory for matched lead set tolerances.
STRAIGHTNESS
Although PowerTrac™ Ball Screws are manufactured from straight,
cylindrical material, internal stresses may cause the material to
bend or yield. When ordering random lengths or cut material without
end machining, straightening is recommended. Handling or machining
of screws can also cause the material to bend or yield. Before,
during and after machining, additional straightening is required.
When ordering screws with machined ends from
Nook Industries, the following straightness tolerances can be expected:
PowerTrac™ SRT and XPR Ball
Screws are straight within 0.010 inch/foot when shipped
from the factory, and do not exceed 0.030 inch in any 6 foot section.
PowerTrac™ SGT Ball Screws
are straight within 0.001 inch/foot when shipped from the factory.
LIFE
A ball screw assembly uses rolling elements to carry a load similar
to an anti-friction (ball) bearing. These elements do not wear during
normal use. Therefore, ball screw life is predictable and is determined
by calculating the fatigue failure of the components. Proper lubrication,
regular maintenance, and operation within specified limits will
allow PowerTrac™ Ball Screws to operate to the predicted life.
EFFICIENCY
The low coefficient of friction of the rolling elements of PowerTrac™
Ball Screws and Nuts results in an operating efficiency greater
than 90%.
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BACKDRIVING
Normally, ball screws are used to convert rotary motion into linear
motion. Backdriving is the result of the load pushing axially on
the screw or nut to create rotary motion. All ball screws, due to
their high efficiency, will backdrive. The resulting torque is known
as “backdriving torque” and is the torque required to
hold a load in position.
CAUTION - When using ball screws, applications
should be analyzed to determine the necessity of a brake, especially
when the possibility of injury may occur.
BACKLASH
Backlash (lash) is the relative axial movement between a screw and
nut without rotation of the screw or nut. Backlash information for
PowerTrac™ Ball Screws and Nuts is listed within the data
section of
this website. Lash in ball screws will remain constant during normal
use.
SELECTIVE
FIT
The axial movement between a new PowerTrac™ SBN or SGN ball
nut and screw will range from 0.003" to 0.015" depending
on size. When less than standard lash is desired, SBN and SGN ball
nuts can be custom-fit to a specific screw with selected bearing
balls to minimize lash to 0.003" to 0.005" depending on
ball size. Select fitting may result in lower life.
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| LOAD
DEFINITIONS |
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STATIC
LOAD
The maximum thrust load – including shock – that can be
applied to the ball nut without damaging the assembly. DYNAMIC
LOAD
The thrust load in pounds which, when applied to the ball nut and
rotating screw assembly will result in a minimum life of 1,000,000
inches of travel. Metric screw designs are per ISO 3408 and show
the load ratings in kilonewtons for 1 million revolutions of the
screw. For inch or metric rotating nut designs, contact Nook Industries
at 800-321-7800.
TENSION
LOAD
A load that tends to “stretch” the screw. (SEE FIG.
3)
COMPRESSION
LOAD
A load that tends to “squeeze” the screw. (SEE FIG.
3)
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| FIG.
3 |
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THRUST
LOAD
A load parallel to and concentric with the axis of the screw. (SEE
FIG. 4)
OVERTURNING
LOAD
A load that tends to rotate the nut around the longitudinal axis
of the screw. (SEE FIG. 4)
SIDE
LOAD
A load that is applied radially to the nut. (SEE FIG. 4)
|
| FIG.
4 |
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PRELOAD
Preload is an internal force introduced between a ball nut and screw
assembly that eliminates free axial and radial lash. Preloaded assemblies
provide excellent repeatability and increased system stiffness. Preloading
is achieved either by using two nuts and forcing them apart or by
shifting the circuits within a single nut. Nook Industries has a variety
of preload ball nut designs available. (SEE FIG. 5) |
| FIG.
5 |
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DESIGN
CONSIDERATIONS |
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MOUNTING
AND PINNING OF BALL NUT FLANGE
If a flange is used, it must be permanently fixed to the nut. Since
mounting methods usually require the disassembly of the ball nut
from the screw, it is best to order the nut and flange factory assembled.
The preferred method of locking a flange to a nut is a pin or set
screw parallel to the screw which intersects the flange/nut mounting
thread. Because of the dissimilarity of materials, the hole may
need to be milled, not drilled.(SEE FIG. 6 & 7)
Alternatively, the flange may be drilled and
tapped radially for a setscrew. After assembly of the flange to
the nut, spot drill the nut threads through the flange and install
a dog point set screw from the flange O.D. into the nut O.D. threads.
Avoid getting metal chips in the nut when drilling.
Commercially available thread adhesives may
be used for light load applications. Follow the manufacturers recommendations
to ensure a satisfactory bond. Avoid getting the adhesive onto the
ball tracks.
|
| FIG.
6 & 7 |
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STANDARD
FLANGE ORIENTATION
Standard flange orientation varies with the number of holes in the
flange. Unless otherwise specified, a factory-assembled flange will
be oriented on the nut as shown. (SEE FIG. 8)
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| FIG.
8 |
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PROPER
BALL NUT ORIENTATION
When a ball screw assembly is used in an orientation other than
vertical, it is important to orient the return tubes to optimize
ball nut operation. (SEE FIG. 9)
Ball nuts run best with the return guides
up. Horizontal guide orientation is also acceptable. Return guides
down should be avoided. Some ball nut designs have return guides
on both sides of the nut. In this case the preferred guide orientation
is horizontal.
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| FIG.
9 |
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| TRANSFERRING
BALL NUTS FROM SHIPPING ARBOR
When not ordered as part of an assembly, ball nuts are shipped on
arbors. Transferring the ball nut from the arbor to the ball screw
is achieved by placing the arbor against the end of the screw thread
and carefully rotating the ball nut onto the screw from the arbor.
If the inside diameter of the arbor is too small to slip over the
outside diameter of the journal, apply tape to the journal to bring
the outside diameter up to the root diameter of the screw to prevent
the bearing balls from falling out of the ball nut. The ball nut
can then be transferred across the taped journal onto the ball screw.
(SEE FIG. 10)
CAUTION - Removal of the arbor from
the ball nut will result in the loss of the bearing balls. All of
the bearing balls in a ball nut are matched. If any balls are lost
during this transfer, they all must be replaced.
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INSTALLING
SEL, SAR, AND SAG BALL NUTS
These nuts must be transferred from the
arbor to the screw without preload. Be sure to keep the ball return
tubes aligned with each other and make sure the coupling tangs line
up with the slots in the ball nut.
Center the adjusting nut on the coupling.
Before preloading these ball nuts, all the coupling threads, spring
washers/spacers and ball grooves should be lubricated.
Position the ball nut on the center of the
screw shaft. It is a good idea to place retainers (tape, tie-straps,
etc.) on the screw to prevent the ball nut from over-traveling.
With the ball return tubes facing upward, tighten the adjusting
nut against the spring washer or spacer by hand until it cannot
be turned. While holding the ball nut with tubes facing up, rotate
the screw several turns in both directions.
Running torque can be measured by means of
a spring scale. The force reading multiplied by the lever arm length
yields the running torque value.
Make adjustments to achieve desired preload
and recheck running torque value up and down the screw shaft. Do
not tighten the adjusting nut to a point that fully collapses the
spring washers. After the system is adjusted, secure the adjusting
nut with the set screws provided.
LUBRICATION
Proper and frequent lubrication must be provided to achieve predicted
service life. A 90% reduction in the ball screw life should be anticipated
when operating the nut and screw without lubricants.
Standard lubrication practices for antifriction
bearings should be followed when lubricating ball screws. A light
oil or grease (lithium-based) is suitable for most applications.
Lubricants containing additives such as molydisulfide or graphite
should not be used.
E-900, Nook Ball Screw Lubricant, is oil that
has been developed specifically for ball screws and is available
as a spray or liquid. Lubrication intervals are determined by the
application. It is required that screw assemblies are lubricated
often enough to maintain a film of lubricant on the screw.
DRIVING
TORQUE
Driving torque is the amount of torque required by the ball screw
to move a load. To simplify this calculation a “torque to
raise one pound or one kN” value is provided in the technical
data for each ball screw size. (SEE FIG. 11)
To determine the required torque to move a
load, multiply the load to be moved by the “torque to raise
one pound or kN”. For more information on drive torque, see
the application selection example. |
| FIG.
11 |
|
WHERE:
Td = Drive Torque (pound-inches)
P = Load (lbs.)
L = Screw Lead (inches/turn)
e = Ball Bearing Screw Efficiency (90%) |
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BACKDRIVING
TORQUE
Due to the efficiency of a ball screw, a load applied to the ball
nut will generate backdriving torque on the ball screw. The torque
required to hold the load in position can be calculated by the following
formula. (SEE FIG. 12) |
| FIG.
12 |
|
WHERE:
Td = Drive Torque (pound-inches)
P = Load (lbs.)
L = Screw Lead (inches/turn)
e = Ball Bearing Screw Efficiency (90%) |
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TEMPERATURE
PowerTrac™ Ball Nuts will operate between -65°F and 300°F
with proper lubrication. PowerTrac™ ball nuts equipped with
elastomeric wipers are limited to operation between -20°F and
180°F.
END
MACHINING
To obtain optimum performance of your ball screw assembly, it is
recommended that the machining be performed at the Nook Industries
factory. Screws may be purchased machined to your specifications
or to standard end machining designs. Annealed ends can be provided
on SRT screws to facilitate end machining of journals.
EZZE-MOUNT™
Ball screws in operation generate an axial load and a radial load;
therefore, end mounts must be designed to accommodate these loads.
Nook Industries has designed precision end mounts to work specifically
with lead screws. For a detailed description of these bearing supports
see the EZZE-MOUNT section. An EZZE-MOUNT™ can be shipped
pre-assembled to a PowerTrac™ Ball Screw.
OPTIONAL
SURFACE COATINGS
PowerTrac™ Ball Screws are available with optional corrosion
resistant and/or lubricated finishes like Nickel, Teflon, or Hard
Chrome; consult Nook Industries for detailed specifications.
WIPER
KITS
It is recommended that wipers be used with ball nuts to prevent
contamination from foreign materials. The product pages detail the
different types of wipers available for or standard with each ball
nut. Brush wipers may require customer-supplied retention. For the
different ways that this can be achieved (SEE FIG. 13). |
| FIG.
13 |
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BOOTS
AND BELLOWS
For contaminated environments, use of a boot or metal cover to protect
the ball screw assembly is recommended. |
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The selection of the correct ball screw and nut for a particular
application involves five interrelated factors. Before attempting
to determine the ball screw and nut combination, the following values
must be known:
- Load measured in pounds or newtons
- Speed measured in inches or millimeters per minute
- Length between bearings measured in inches or millimeters
- Life Expectancy
- End fixity type
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LOAD
The loads that need to be considered are the static loads, dynamic
loads, reaction forces and any external forces affecting the screw.
See Load definitions section above for details.
SPEED
The travel rate (linear speed) is the rpm at which the screw or
nut is rotating multiplied by the lead of the screw.
LENGTH
The unsupported length of the screw.
LIFE
EXPECTANCY
The dynamic load ratings shown on the product specification pages
indicate the load that can be carried for 1,000,000 inches of travel.
The charts on pages 90-91 relate life to load. In applications where
the load is relatively constant over the entire stroke, use the
highest load to select the ball screw to provide a factor of extra
life. For applications where the loads vary significantly, an equivalent
load can be calculated using the following formula:
|
|
WHERE:
Lm = equivalent load
Ln = each increment of load
%n = percent of stroke at load Ln
FOR EXAMPLE:
L1 = 150# % = 30%
L2 = 225# % = 45%
L3 = 725# % = 25%
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The life required is determined by multiplying the total stroke
in inches by the total number of strokes required for the designed
life of the equipment.
To calculate the travel life for a ball
nut other than at rated load use the formula (SEE FIG. 14). |
| FIG.
14 |
|
|
WHERE:
Tx = Travel other than rated load. Life is given in inches or meters
Fr = Rated Load in pounds or kilonewtons
Fx = Actual or Equivalent load in pounds or kilonewtons
Tr = Rated Travel Life. For inch screws this
is equal to 1,000,000 inches. For Metric Screws this is equal to
the ball nut lead in meters times one million revolutions.
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END
FIXITY
End fixity refers to
the method by which the ends of the screw are supported. The degree
of end fixity is related to the amount of restraint of the ends of
the screw. Examples of the three basic types of end fixity are: |
| FREE |
No support. |
|
| SIMPLE |
Shaft supported at a single
point. |
|
| FIXED |
Shaft rigidly restrained
against axial rotation. |
|
|
Simple End fixity can be provided through a single
bearing support. Multiple or spaced pairs of bearings are more rigid
than a “Simple” support, but because of their compliance
are not truly “Fixed”. A screw can be supported with
different combinations of end fixity. (SEE FIG. 15: A – D)
A: One end supported with a Double Bearing EZZE-MOUNT™, other
end Free. Use Line “A” in reference to the charts shown
for Column Strength and Critical Speed
Charts. NOTE: Not recommended for any application other than
short travels and slow speeds.
|
| FIG.
15 A |
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B: One end supported with a Double Bearing EZZE-MOUNT™,
other supported with a Single Bearing EZZE-MOUNT™. Use Line
“B” in reference to the charts shown for Column
Strength and Critical Speed Charts.
|
| FIG.
15 B |
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C: Both ends supported with a Double Bearing EZZE-MOUNT™.
Use Line “C” in reference to the charts shown for Column
Strength and Critical Speed Charts.
|
| FIG.
15 C |
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D: Both ends rigidly mounted with rotating nut or both
ends mounted with a double preloaded angular contact bearing spaced
apart by least 1.5 time the diameter of the mounting journal. Use
Line “D” in reference to the charts shown for Column
Strength and Critical Speed Charts.
|
| FIG.
15 D |
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CRITICAL
SPEED
The speed that excites the natural frequency of the screw is referred
to as the critical speed. Resonance at the natural frequency of
the screw will occur regardless of the screw orientation (vertical,
horizontal etc.) or if the system is designed so the nut rotates
about the screw.
The critical speed will vary with the diameter,
unsupported length, end fixity and rpm. Since critical speed can
also be affected by shaft straightness and assembly alignment, it
is recommended the maximum speed be limited to 80% of the calculated
critical speed. The theoretical formula to calculate critical speed
in rpm is: |
|
WHERE:
N = Critical Speed
d = Root Diameter of Screw
L = Length Between Bearing Supports
Cs = .36 for one end fixed, one end free
1.00 for both ends simple
1.47 for one end fixed, one end simple
2.23 for both ends fixed
The Critical Speed
Chart is provided to quickly determine the minimum screw size
applicable for Nook EZZE-MOUNT™ designs.
Maximum travel rate is also limited by ball
velocity. The ball velocity is a function of the ball circle diameter
and rotational speed. Ball velocity is limited by a maximum DN (ball
circle diameter x rpm). The charts show the maximum speed based
on the DN value for each screw in parentheses. If the selected ball
screw does not meet the speed criteria, consider the following options:
a) Increase screw lead (reduce rpm)
b) Change end fixity (e.g. simple to fixed)
c) Increase ball circle diameter
The final consideration should be to recheck
the selected screw against all three of the design criteria: life,
column strength and critical speed.
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COLUMN
STRENGTH
When a screw is loaded in compression (see
compression load definition above), its limit of elastic stability
can be exceeded and the screw will fail through bending or buckling.
The theoretical formula to calculate the column strength in pounds
is:
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|
| WHERE:
Pcr = Maximum Load
Fc = End Fixity Factor
.25 for one end fixed, one end free
1.00 for both ends supported
2.00 for one end fixed, one end simple
4.00 for both ends rigid
d = Root Diameter of Screw
L = Distance between nut and load carrying bearing
The Column Strength
Chart may be used to verify that the screw can carry the required
load without buckling.
The charts show the theoretical limitations
of each screw on a separate line. The lines are limited horizontally
by the slenderness ratio and vertically by the maximum static capacity
of the nut. Actual load is limited by maximum nut capacity.
If the selected acme screw does not meet compression
load criteria, consider the following options:
a) Change end fixity (e.g. simple to fixed)
b) Design to use screw in tension
c) Increase screw diameter
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