The task of selecting lead screws can often be confusing because the many variables involved. Lead, rpm, critical speed, diameter, columns strength, and efficiency all depend on one another in some form. So how do you choose a lead screw to meet your needs?
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The following section will guide you through the steps of choosing a lead screw. This is an accepted way of narrowing and choosing a lead screw in many industries.
Although recommended, you do not have to follow the steps in order described, but always complete each step before deciding on the lead screw. Keep in mind that these steps will have to be performed for each axis utilizing a lead screw system .
Also keep in mind that you may have to revisit this section after the motor selection. Lead screw selection and motor selection really go hand in hand, but it usually works best to understand CNC drive parts first.
Requirements before Proceeding (covered in proceeding sections)Step 1: Determining the forces acting on the lead screw
Step one involves determining the loads on the lead screw by using the weight of the assembly to be moved by the lead screw, the linear bearing type, the velocity, and the desired acceleration. It is important to know what kind of loads will be applied to the lead screw to check for column strength issues. Also, you will be able to use these loads with you lead screw selection to calculate required torque.
Click here to read how to read how to calculate these forces manually.
OrUse the lead screw forces calculator here.
Step 2: Find Minimum Diameter of Lead Screw
The next step involves finding the minimal diameter of a lead screw given the maximum distance between the nut and the farthest end bearing, the maximum load, end fixity type, and the slender ratio. The slender ratio is the ratio of the minor diameter of a lead screw to the lead screw length. A lead screw may only be so long in respect to the diameter before failure is imminent.
Use this calculator to find the smallest diameter lead screw that will work for your application. Use the total load that was calculated above.
It should be noted that this calculator will provide the absolute smallest minor diameter of a lead screw that will be safe given the span and end fixity type. You will need to find the closest fractional size offered by your desired manufacturer.
You may also use the column strength calculator if you have preselected a diameter and span and need to check if the column strength is more than the applied load.
Step 3: Selecting Lead
Above we selected a lead screw that is as close to the minimal diameter that was calculated. Now we must select the lead screws lead.
Here is where things get a little trick and may take a few iterations. Because changing the lead affects the linear resolution and the force output of the drive, you may need to revisit this section after finalizing your motor selection. For now, here is the recommended approach.
First see what leads are available in the diameter selected. Don’t forget to check lead screw nut availability and choose one (higher leads are usually better”. Next:
Use the Linear speed calculator to see how many RPM the screw must do to achieve the desired linear speed.
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Step 4: Check critical Speed
Now that you used the Linear speed calculator to see the maximum RPM, use that RPM value to place into the critical speed calculator below.
Use the Critical Speed calculator to check see if the given lead screws diameter and length allows for sufficient RPM of the lead screw.
If the critical speed is higher than the operational RPM, choose a larger diameter, shorten the lead screw, or choose a higher lead.
Run through these steps a few times to try and choose the best solution for your router. Often times we must compromise on what we want to meet our budget or what is available. However, use these tools to help make sense of all the numbers.
Play around with end fixity type, diameter, lead, and other attributes to see how it changes the cost of your setup. You can often trade attributes and still reach a similar outcome.
When engineers come to me looking to utilize a lead screw to actuate their application, I get them thinking about four main data points that can help determine the majority of questions I need to ask to help them utilize lead screws and polymer nuts properly. Those data points are payload, speed, duty cycle and stroke.
In this blog post, I'll dive deeper into each of those data points and discuss additional factors that you should consider when choosing the right lead screw for your application.
The first step is determining the quantified axial payload. This figure helps determine how much force is required to drive the application.
Second, you want to take the application's speed into account. I usually ask an engineer a certain set of questions to help determine what the goal is for the application. It could be as simple as how many seconds do you want to complete this stroke in? Or, how quickly does it need to get from point A to point B?
The RPM (feed rate / lead of desired screw) is important but is typically determined after you have figured out the application’s linear feed rate, also known as the linear distance traveled per minute. Once you have that figure, you can determine the RPM of the application by taking a look at your desired lead screw and noting the distance traveled per revolution.
When working with a polymer lead screw nut, one of the more important factors that's going to determine the lifetime of the part is the application’s duty cycle. How often is it going to be running per minute, per hour, per day?
This is extremely important for a polymer part because if the application has a higher load and a higher speed, the wear is going to be higher for the part. So, to properly spec a lead screw size, you need to take duty cycles into account to make sure that the PV values are in check.
Also, make sure you ask yourself how long is the application stroke and how long does the overall screw need to be?
Payload, speed, duty cycle and stroke are four extremely important data points; but that's not all that should be considered.
You should be asking yourself whether or not you need a lead screw that's going to be self-locking or if you want a part that is able to back drive. You should be thinking about the types of linear guides that you're using for an application. Are you going to be using a sliding guide? Or, are you going to be using a rolling guide, like a reciprocating ball bearing? Those types of questions are going to help determine whether or not you can use a smaller or a larger lead screw in your application.
You also want to take into account how you're going to drive this application. Are you going to be using a DC motor or a stepper motor? Are you going to be hand cranking this application? These are some of the secondary factors that you want to consider when you're trying to narrow down what lead screw you use in your application.
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