Welcome to Misumi's New 19-minute Webinar Series.
For those of you here that are new to Misumi,
Misumi is a global supplier of Configurable Components.
You may ask, why configurable? What is it?
Configurable components solve the designer's dilemma.
A designer has two choices for components; Standard
or Custom. Standard components have a limited selection
and often forced designers to compromise their designs.
These many pluses such as CADs, static part numbers,
fast delivery, often seem inexpensive
and convenient. The other choice is Custom.
Custom components have many drawbacks.
Designers have to make the solid models themselves and all the manufacturing
drawings.
Purchasers too can attest to the long wait for quotes,
the expensive set up charges for short runs, and the long lead times.
We are forced to use custom components though because of the flexibility.
Misumi offers Configurable,
which is the best of both; easy and flexible.
Configurable components are easy to design with and purchase.
Online, select the product geometry desired,
the material and surface finish, input the size for each configurable dimension
within the range and increments shown.
You'll get a smart part number, instant price and days to ship,
and the CAD model. All of this configurability
is available with our over one million components
available at misumiusa.com.
Now I'll turned it over to our Misumi engineer.
Hi, my name is Vince Chan. I
am a product engineer for standard components. In this webinar,
I will introduce to you an actuation component call Ball Screw.
What are ball screws? They are another kind of mechanical actuators that can
translate rotary motion into linear motion.
The ball screw is made up by a rotating screw shaft
and a ball nut. The ball nut is a sub-assembly consisting of recirculating ball
bearings.
The bearings roll in and out of the nut, then along the grooves,
which minimizes the friction of the motion. Motion follows the same principle
as the bolt
and nut. As the shaft rotates the nut moves linearly along the screw shaft.
The ball nut has a flange. The flange is for mounting brackets
and they will typically face the fixed side of the shaft.
Ball screws are essential components for linear motion systems,
such as motor-driven actuators, often found in precision applications.
However, ball screws are not the only components that can convert
rotary motion into linear motion. There are also belts and pulleys,
gears and gear racks, or lead screw nut assembly.
This last in particular also follows the principle of the ball and nut.
Therefore, why use ball screws? Because they offer several advantages,
one of which is low friction. Compared with lead screws that have high friction
loss,
the rolling coefficient of friction for ball screws is as low as .002.
As a result, high energy efficiency can be achieved and virtually no stick-slip
would occur. They are accurate and precise.
Their accuracies can be 12 microns or less within the target displacement.
The accuracies of Misumi ball screws are controlled by the Japanese
Industrial Standard,
abbreviated as the JIS. Ball screws have little to no backlash.
Backlash is a term for lost motions caused by
gaps between mechanical parts. It can negatively affect the repeatability of
the travel.
Because ball screws can be pre-loaded to high rigidity,
they can achieve practically zero backlash.
Last but not least, they are high quality products.
Each individual component is hardened to at least 56 Rockwell C
by induction or by carburization.
The ball screw nut works by recirculating bearings that roll along the
grooves
of the shaft. There are four basic types.
In the first method, bearings are recirculated through the end cap on each
end of the nut. This type performs well at high speed operations.
The second method invests deflectors on the inside of the nut to return the balls
over the screw shaft. This method is suitable for compact designs.
The third type uses return tubes mounted on the outside of the nut to
recirculate the bearings.
This is an easy to assemble method suitable for mass productions.
The last method uses guide plates instead of return tubes for recirculation.
The advantage here is also compact design.
There is not a standard method. The recirculation type depends on the
specification
of the ball nut. For example, larger standard nuts may have return tubes,
while smaller compact nuts may use deflectors.
Ball screws achieve high rigidity and repeatability through preloading.
There are three common ways. The first way is to create a constant force by using
bearings that are slightly larger than the size of the grooves.
The second way is to use a spacer between a set of nuts
with thickness precisely machined to achieve the desired preload.
And the third method utilizes spring and double nut configuration.
The precision class ball screws are typically preloaded
and therefore have much finer repeatability than non-precision classes.
Misumi ball screws are made according to the JIS
Accuracy Classes. It is one of the standards that define the accuracies
of the ball screw. The accuracy class is denoted by the letter C,
followed by a number representing the accuracy grade.
The smallest C number that it can start with is 0.
As the C number increases, the accuracy decreases.
For example, C0 is more accurate than C3,
C3 is more accurate than C5, and so on.
The lower the accuracy the greater the error to be expected.
The choice of Accuracy Class depends on the application.
For example, an industrial precision robot may require C3,
while a woodworking machine may be sufficient with C7
or C10. When selecting ball screws
you must know that for a given distance of travel, what amount of deviation will
you tolerate?
The Accuracy Class defines the expected errors for the ball screw.
There are four major values that the JIS controls.
Ep is the mean difference between the intended travel
and the actual travel. Vu--
the range of variations over the actual travel.
V2 pi is the variation of the actual travel
from an arbitrary full revolution.
And V300 is the variations from the actual travel
from an arbitrary 300 millimeter span.
To show you an example of the difference in accuracy going from one class to
another,
compare between the C3 and the C5 ball screw in this table
at a length of 800 millimeters. The C5 exhibits 17 microns more
error in the actual mean travel
and 12 microns more error in the travel variations.
In general, the amount of errors to be expected is about twice more
in C5 than C3.
Misumi offers two precision grades of ball screws.
They are the precision ground and the rolled ball screws.
The precision ground have JIS classes of
C3, C5, and C7, while the rolled types have C7
and C10. As you can see, the precision ground has higher accuracy.
The difference between the precision ground and the rolled screw is that
on the precision grade, the grooves are finished by machine grinding,
whereas on the rolled grade the grooves are co-rolled.
The first method ensures precise concentricity with the ball thread.
In addition, the precision ball screws are pre-loaded to insure high repeatability.
The choice of precision grade is dependent on the application.
Suppose you have an application where you need to position the mass
anywhere along a given distance. Some of the important constraints are; speed,
accuracy, repeatability, design life,
and motor speed. The values listed here are examples for demonstration purpose
only. The ball screw will be mounted to a motor on the fixed end,
while simply supported on the other end.
This is a general guideline to selecting the ball screw.
First, calculate the lead, then the minimum dynamic load rating,
and then finally the total length.
Step 1: calculate the lead. A lead
is defined as the displacement of the ball nut in one rotation.
The lead can be calculated by dividing the speed,
Vmax, by the maximum motor revolution,
Nmax. In the example given,
the minimum lead required is 20 millimeters.
Step 2: find the minimum load rating.
There two load ratings; the dynamic load
and the static load. The static load is the maximum allowable load while in
steady state.
On the other, hand selecting the dynamic load rating is more involved.
The dynamic load rating is an accumulative load value
that has accounted for the dynamic forces of the motion.
It is defined where ninety percent of the ball screw will last at least 1
million rotations
without wear of the rolling surfaces.
The dynamic load
rating C can be calculated as a function of the operating life,
Lh; mean rotation rate Nm;
Mean Axial, Pm; and the work factor,
fw. This last parameter
accounts for sudden impact in operation. It ranges from 1 to 2,
from no shocks to frequent impacts. Let us assume a work factor of 1.2 for this
example,
but in order to acquire the three remaining variables
you must know the motion profile of the application.
An example of motion profile
would be a graph of velocity versus time.
From it you can extract the velocities and accelerations of various
durations of the cycle.
Using this information in conjunction with formulas available in the data
section of the catalog,
you can calculate the mean axial load, mean rotation frequency and
actual operating life. The actual operating life may not be the same as
the design life.
Calculate the actual based on the fraction of the cycle when the unit is
actually in motion. Once you have determined the required variables;
Pm, Nm, Lh, and work factor,
plug them into the dynamical formula to calculate the minimum
dynamic load rating required for ball screw. In the example given I've calculated
3700 Newtons.
If you would like to be more conservative you can always adjust the
individual variables.
In the last step, calculate the total length.
It is not simply the stroke, it also includes the length of the ball screw nut,
shaft end lengths, and an allowable margin of travel at either end of the stroke.
Be sure to review the catalog drawings to determine the total
you need. Accounting for all of these parameters,
our example would need a minimum of 914 millimeters.
At this point you should have all
information needed, namely the lead, dynamic load rating,
total length of the shaft, and the accuracy and repeatability for the application.
As you browse through the Misumi catalog, you will find that the BSS product type
meet or exceed all of the requirements specified by this example.
If you check the accuracy of the C5 class ball screw,
you will see that it falls well within the requirements of 50 micron accuracy.
You can begin generating a part number from this selection.
Misumi ball screw part numbers are coded based on material,
shaft diameter, lead, and configurable length.
In this example we have BSS 1520-914.
Misumi catalog offers three types of ball screw nuts.
They are the Standard Nut, Block Nut, and Compact Nut.
The compact nut is unique in that it can have up to forty percent reduction
in volume
compared to standard ball nuts. As a result, you can design lower-profile units,
but because of reduced volume they will recirculate less ball bearings and
therefore have smaller load ratings.
Not every grade of accuracy and precision is available for each type.
Grade C3 through C10 are all available for the standard type
in both rolled and precision grades. The block type is available in C10
rolled grade.
The compact nut is available in C3 and C10 accuracies in both precision grades.
Be sure to check the Misumi website for the latest product availability.
Misumi offers configurable dimensions to the ball screw
shaft. In this drawing, all the dimensions in blue,
namely the shaft length and diameters, and end lengths, are
all configurable in one millimeter increments.
In addition to standard configurations, several alterations are available.
Alterations are optional changes to the units such that it may
ease the installation of the ball screw shaft. To name a few,
these include reversing the direction of the ball screw nut
so that the flange will face the opposite end of the shaft.
Ordering without Retaining Ring Groove, adding wrench flats,
adding keyways, installing a temporary shaft to the ball screw nut so that
additional machining can be done prior to installation, or
ordering the shaft without any standard machining.
Also available in the catalog are a variety of accessories that will ease your
search of components.
There are various bearing support units for both the fixed side,
and the support side. You will also find peripheral accessories
such as nut stoppers, nut brackets, and joint units
for AC servo motors. Be sure to check the online catalog for the latest
components.
Please keep in mind that these products are very high quality and competitively
priced.
In-stock items are shipped the same day, while non-stock items are shipped in a
minimum of four days.
Misumi ball screws are initially filled with lithium soap grease.
You may need a different grease due to environmental constraints.
The first alternative available is an L-type ether synthetic grease for
high-temperature
operations. The second option available is the G-type mineral grease for
cleanrooms that require
minimal particle dispersion. When contaminated with dust,
ball screw accuracy can diminish. It is recommended to re-grease
every six months or every 1,000 kilometers.
Some ball screws nuts have oil access ports, allowing you to directly
apply the grease, However, for those without the oil access port,
apply the grease to the shaft grooves then rotate the nut back along the shaft.
In general, proper selection and maintenance will ensure a long life and
accurate performance of the ball screw. Let us go to the Misumi USA website
and practice selecting a ball screw using the online configurator.
Here's the Misumi home page. It is divided into three sections;
Automation, Pressed Die, and Plastic Mold. We will focus
on automation components.
In this page, you can browse the major categories
by the left panel or the pictorial indexes
on the right. Under the linear motion category,
find and click on ball screw.
Here you can narrow down your search using the filter on the left
or you can browse through the list on the right. For this example I will select
in the filter column
twenty-millimeter lead, fifteen millimeter OD,
and C5 precision. Now there's only one
option to select.
I click on it and the site redirects me to the details page.
In the filter, I will select the only options available to me,
then I will manually type in the length of the shaft.
For the grease type I will go with standard.
Since all necessary values have been entered,
the site automatically generated the part number.
To view the alterations available for this part,
scroll to the bottom of the main page. To enter the alterations,
scroll down in the filter section. For now we're skipping the alterations.
To find compatible support units for this part,
click on the More Information tab and you will find recommended support
units
or any applicable accessories. This concludes our
online presentation. Thank you.
We hope you enjoyed today's webinar. All of Misumi's configurable components
and one million products are available
at www.misumiusa.com.
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Full support is also available:
technical help at engineering@misumiusa.com,
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