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 turn it over to our Misumi engineer.
My name is Mike Malone, and I am an engineer
at Misumi. Today we'll be talking about Lead Screws. We have a lot to cover so
let's get started.
The topics to be covered are: definitions, their advantages,
Misumi product offerings, and application examples.
At the end we will cover how to find lead screws on the Misumi website,
and how to download CAD. Let's get started. At its most fundamental level,
a lead screw screw is simply a threaded rod and a nut, but instead of being used for
fastening,
it is used to translate rotary motion into linear motion. Due to this factor,
it is a key difference in the design of a lead screw as opposed to a threaded rod.
This difference is the thread form that is used. Lead screws don't use the
Unified Thread Standard, or
ISO metric screw thread, as those forms are V-shaped.
The V shape design produces more friction between the nuts and the shaft,
which is desirable in fastening as it helps prevent the nut from loosening.
But, as a lead screw is not used for fastening, any extra friction would not be
desirable.
They utilize a couple of different thread forms. These include but may not be limited to:
square thread, buttress thread, trapezoidal threads.
Following is a brief description of each. Square threads are the most
efficient thread form as they have the least amount of friction.
As a result, the square form is often used in high-power applications.
The drawback of the square form is that it is the most difficult form to manufacture,
thereby making it the most expensive. Buttress threads are effectively square
threads with half of the thread cut off, changing the rectangle to a triangle.
This form can only be used in applications where the force is applied
in only one direction.
This thread is used because they are easier to manufacture than square threads,
but they have the same efficiency. Trapezoidal threads are
easier to manufacture than square threads, and can accept loads from both
directions.
But they're less efficient than square threads, and because they are easier to
manufacture and can accept loads from both directions,
trapezoidal threads are very popular on lead screws. They can also handle large
axial forces compared to square threads
due to the fact that they have comparatively large root. In the industry, there are two
very popular forms of trapezoidal threads;
ACME, and Trapezoidal Metric. The ACME thread form has a twenty-nine degree
thread angle,
with a thread height that is half of the pitch. This standard has been used quite
extensively for a long time in the industry.
Thread diameters are defined in nominal inch sizes.
Metric trapezoidal threads are very similar to ACME threads except the
and is 30 degrees instead of 29 degrees,
and the thread dimensions are in millimeters instead of inches. It goes
without saying that, while similar,
ACME threads and metric trapezoidal threads are not compatible with each other.
It is this thread form that Misumi uses on all of their lead screws.
Lead screws and ball screws are very similar components. Both convert rotary
motion into linear motion in effectively the same way.
The difference between the two is that, while the lead screw has the nut slide
over the screw shaft,
the ball screw has the nut roll over the screw shaft on a recirculating ball bearing
mechanism.
This has its advantages and disadvantages. Relying on rolling
friction instead of sliding friction makes ball screws extremely efficient
and allow for tighter tolerances including preloading, resulting in accurate
movement.
It also allows for a predictability of its life span in use in high-speed
applications,
but the recirculating ball mechanism adds a great deal complexity to the
manufacturing process,
making them more expensive. They can also generate more noise in use.
This leads to a very important question: why us a lead screw as opposed to a ball screw?
The answer is, it depends upon the application. In general, there are couple
main advantages to a lead screw.
The most obvious is cost. In OEM situations where unit cost needs
to be minimized,
lead screws are very popular. So while in some situations there may be more work
done in the design stage in terms of testing and tweaking,
that extra cost is outweighed by the lower cost of the lead screw on a
component level.
So on an OEM level, lead screws are used quite extensively as opposed to ball
screws,
as a way of minimizing unit cost. Second, because of the fact that there are no
moving components inside of the nut,
lead screws are very smooth and quiet in operation. Many balls screws do not have
individually caged balls, resulting in a noticeable amount of noise during use.
On the other hand, in most situations, when properly installed,
lead screws cannot be heard during use. Another benefit of not having ball
bearings inside of the nut is that the lead screws can be used in corrosive
and dirty environments. Because lead screws don't have hardened ball bearings
coming into direct contact with the surface of the thread,
it is not required that the shafts be hardened. This allows for the use of
other less corrosive materials for both the shaft and the nut.
For example, the shaft can be made out of a non-harden-able 303 stainless steel
instead of the harden-able, but much more susceptible to rust,
440C stainless. The nuts don't even need to be made out of metal.
They can be made out resins like PPS or polyacetal.
Some combinations of shafts and nut material can even be run without grease,
which is useful in dirty environments. Also,
many lead screws don't back drive. That means applying an axial force to the nut
does not result in rotary motion.
This is very useful in vertical applications in that a locking
mechanism is not required.
Once the nut is moved into position, it will remain there
even in the power outage situation. And finally,
lead screws are a very simple component. Because there are no moving parts inside
of the nut, it is not dependent
upon any internal mechanisms working correctly in order to function.
Quite simply, simple design means less that can go wrong with it.
Misumi sells several different types of lead screws;
Straight, One End Stepped, One End Double Stepped,
One End Stepped One End Double Stepped, Both Ends Double Stepped,
and pre-configured for use with our support units.
The shafts are available in either 1045 carbon steel with an option getting a
black oxide surface treatment or low temperature black chrome plating,
or 303 stainless steel. All Misumi lead screws have a metric trapezoidal
thread.
For the vast majority of Misumi lead screws, the pitch of the lead screw is based upon the
thread diameter.
This table lists our the basic specifications for the lead screws that
we sell.
Notice there are two sizes, 16mm and 20mm, that are available in a fine
thread version of two millimeters,
along with the standard pitch version. This is important because most of our
nuts are available
only in the standard pitch. The only nut available for the fine thread is a
flanged brass nut.
This table also lists out the allowable dynamic thrust that the nuts can handle.
Depending upon size and the material, the loads range from 255 Newtons
up to 40 Kilonewtons. For reference,
here are the accuracy standards that Misumi lead screws adhere to.
As for the nuts, Misumi carries a couple different styles.
We carry flanged nuts where mounting holes are parallel with the shaft.
The flange can be round, square, or compact shaped. We carry straight nuts,
and we carry pillow block style nuts where the mounting holes are
perpendicular to the shaft.
For the flanged nuts, we have versions where the mounting holes are through holes
or slots, and
we have a pyolated version where the flange is offset from the end.
And for applications where backlash is a concern, we carry an anti-backlash nut.
This nut consists of two sections that are spring loaded against each other.
This results in the nut threads always resting against one side of the screw
threads,
eliminating backlash. Lead screw nuts are available in a couple different
materials;
brass, brass with a solid lubricant embedded, tribological PPS resin,
and polyacetal. Listed here are some useful technical calculations for lead
screws.
The two important values to pay attention to are the Pressure (P) and the Velocity (V).
Once these values are calculated, a position in the PV graph can be
determined.
If the PV position is below the relevant curve, then no abnormal wear should occur
in operation.
For example, let's take a look at using an MTSRW16 shaft
and an MTSFR16 brass nut, under the operating conditions of 300 Newtons
axial load at 300 rpm. Crunching the numbers gives us a value
of P equals 0.44 Newtons per millimeter squared,
and a V value equal to 22.8 meters per minute.
Locating this point in the graph reveals that no abnormal wear should occur.
Other useful calculations include finding the screw efficiency and load torque.
Using the same example conditions as before, the screw efficiency is
calculated to be 0.24
the load torque to be 29.7 Newtown centimeters.
Misumi offers lead screws in most the popular thread sizes,
from eight millimeters to 50 millimeters. More importantly,
dimensions shown in blue are configurable in one millimeter increments.
This is important, as in many situations it allows you to order a finished lead screw
that can be bolted onto your application without needing any secondary machining
operations. If that's not enough for your application, the lead screws are
available with optional modifications.
We call them alterations. The alterations are available for lead screws
including; set screw flat machining, wrench flat machining,
adding retaining ring grooves, tapping the ends,
threading for bearing nuts, square chamfering,
and adding a keyway. These alterations are not required but are available
if needed. Also keep in mind that you can add as many alterations as you need,
as long as the alterations don't interfere with each other.
And finally, Misumi also carries some accessories that can be used with the
lead screws.
For example Misumi carries Support Units. These are the bearings with housings
that allow the lead screws to be mounted.
Misumi also carries brackets for nuts. These change the mounting surface
direction from parallel
with the shaft, to perpendicular to the shaft.
Also available are Digital Position Indicators. These will count the
revolutions of the shaft and display the distance traveled by the nut
in millimeters.
And finally, Misumi carries Clamp Plates that bolt directly to the support units
or the position indicators. These will clamp down on the screw shaft,
preventing it from rotating. The following are a couple of examples
showing how lead screws are used.
In this application the combination of a hand wheel, lead screw, and
position lock are used to allow manual positioning of the fixture.
In this case, instead of allowing free positioning of the fixture, the use of
a lead screw allows accurate positioning,
reducing the amounts of time required to position it. Once in position, it can then be
locked into place using the hand lock.
In this application, a left and right hand thread lead screw is utilized to
create a pinching type action.
This simplifies the mechanism in that two separate screws are not required,
nor are the components needed to synchronize their motion. All that is required
to achieve this motion
is one lead screw, two nuts, and the support units to support it.
Altogether, this results in a cheap, compact design
that's robust and reliable. In this application,
much like the first application, a lead screw and a hand wheel allow for manual
positioning of the mechanism.
In this case, the combination of low cost and smooth, accurate operation
make the lead screw a good fit for this application. Now we will look at the reference
shoulder adjusting mechanism.
This is a manually operated unit that moves a contact block back and forth.
In an application like this, the rotational speed is very small as the hand wheel is
turned by hand.
Also, as the contact block is coming into contact with the press fit head,
neither the absolute positioning accuracy, nor repeat-ability,
nor backlash is critical. Because of this, a lead screw is a very appropriate
component for use in this type application,
as the added expense of a ball screw is not justified. Shown here is a sheet
steel roll base.
In this application, a lead screw is used to move a support roller with respect to
its mating roller.
Doing such results in changing the height of the roll of the sheet steel.
Again, in an application like this, the rotational speed is small
as the height of the roller does not need to be continually adjusted.
And again, neither absolute positioning, nor repeat-ability, nor backlash, nor
continuous power transmission are critical
in an application like this. As such, the added expense of a ball screw is not
justified.
Not all lead screw applications need to be large.
Shown here is a camera inspection unit. In this case,
a slide screw is used to move a camera assembly up and down.
A slide screw is simply a smaller, less expensive lead screw that Misumi sells,
and as such are perfect for applications like this.
In this application, backlash is not problem as gravity is always loading the nut
against the thread, and the movement of the entire mechanism
is very smooth, resulting in less vibration, which the camera could pick up.
Now let's go to the Misumi USA website and see how to download CAD
models.
We'll start at the top. Misumi's homepage is www.misumiusa.com.
Here you'll see that we actually have
three different product groups: Automation Components, Pressed Die Components,
and Plastic Mold Components. I bring this up because
everything that we've been talking about today is part of our Automation Components
catalog, so we need to go to the Automation Components website.
There are two ways to search this page.
The first way is to use the search box
in the upper left-hand corner of the screen. In here you can type in a part number
or product description, such as a linear shaft
or shaft support.
The other way to search is by using the categories. Down the left hand side of
the screen here we have all our major categories; we also have them listed in
the center
of the screen. Now what we're looking for, obviously, is lead screws, so we will find that
in linear motion
and click on the lead screws. From here we will click on lead screws again and that's
going to take us to
a product listing of all the different lead screws that we offer.
Now from here, on the left hand side, we can filter it down
based on different criteria, but let's just jump straight
into lead screws for support units, just so we can see
how to download a CAD model. At this point, this page displays
all the information we have on our lead screws for support units.
On this page, as you scroll down, you'll see
all of the technical information in regards to it. Now,
to actually get to the CAD data, we need to configure this part down to a single
part number.
This is done simply by making these choices in this configure box in the
left hand side.
For this particular case, let's go ahead and choose a 1045 carbon steel;
right hand thread; nominal thread diameter, I don't know, let's give it a 22;
a pitch of 5 millimeters; let's give it a surface treatment of black oxide.
At this point you'll notice that those other sections that were made were for
all selection boxes, this is actually a text box.
On the right right next to it you'll see you
the available sizes that you can choose. What this is saying is that you can
choose anywhere between 150 and twelve hundred millimeters
in one millimeter increments, I don't know, let's do 176 I guess.
S dimension: now these dimensions are all going to be listed in the center
of the page,
so you'll know what these variables actually mean, so this
S Dimension is the stub lenghth, here, I don't know, let's give it a stub length of
76 I guess, and uh,
that's about it. At this point you'll notice that we are now resolved to a
single part number:
so the MTWBK22-976-F76
is a complete Misumi part number. To see how much this part costs
we can click on the Price Check button,
and we can see pretty quickly, price is $120.17.
At this point, let's go ahead and download the CAD model,
so all we need to do is click on the CAD Download/Preview,
click on the download 2 and 3D CAD.
At this point it's really simple, just click your CAD file format,
let's go ahead and choose a SolidWorks 2013, and just click Generate.
It's really that simple. In a minute or two what we'll see is a CAD download
link,
and once it appears you just click on the link and it'll download your CAD
model.
That concludes our webinar for today. I'd like to thank you all for attending and
I really hope to see you again
at our next webinar. Thanks again.
Thank you, and we hope you enjoyed today's webinar.
All of Misumi's configurable components and one million products are available
at www.misumiusa.com.
If you are new to Misumi, register for access to CAD downloads,
online formal quotes and orders, and manage all of your account histories.
For first-time companies purchasing, you can receive thirty percent off.
Full support is also available;
technical help at engineering@misumiusa.com,
customer service at inquire@misumiusa.com, and your local account manager at sales@misumiusa.com.
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