CNC Milling Application

Application Description

Given the following requirements, select precision ball screw systems for this application.

Step 1: Determine Specifications

  • 5,000 lb load supported and guided on linear bearings moving horizontally
  • 36" travel
  • Complete 36" travel in 10 seconds
  • Bearing Support Undecided
  • Positioning accuracy +/- 1/4"

Step 2: Analysis

Find the Axial Force Required to Move the Load

The axial force is determined by multiplying the coefficient of friction of the guidance system by the load.

  • F = μ x N - (μ = Coefficient of Friction of the guidance system)

Using Nook linear bearings in this application:

  • μ = Coefficient of Friction for lubricated Nook Linear Bearings = .0013
  • N = Load = 5000 pounds
  • μ x N
  • F = .0013 x 5000 lb
  • F = 6.5 lb

Therefore, we can definitively state that the axial force the screw must produce to move the load is 6.5 lb.

Find the Average Travel Rate

The average travel rate is determined by dividing travel distance by the travel time.

  • V avg = D/t
  • D = distance = 36 inches
  • t = total time = 10 seconds
  • V avg = 36 in/10 second
  • V avg = 3.6 in/second or 216 in/minute

Therefore, we can definitively state that the average travel rate is 216 inches/minute.

Find Maximum Travel Rate

When considering critical speed, peak velocity should be used. Using a basic triangular motion profile (acceleration = deceleration with no constant velocity travel), the peak velocity equals twice the average velocity.

  • V peak = 2 x V avg.
  • V avg. = 3.6 in/second or 216 in/minute
  • V peak = 2 x V avg.
  • V peak = 432 in/minute

The Maximum Travel Rate is 432 in/minute during the traverse of 36 inches in 10 seconds.

Determine Total Unsupported Length

Total Travel is given as 36", but extra screw length should be considered for travel nut, carriage, and any extra screw length for over travel. Based on the travel nut and attachment of the nut to the carriage in this application it is determined that an extra 4" of screw length will be required. (Refer to the dimensional information of the particular nut used)

L total = 36 in + 4 in = 40 inches

The total unsupported length to be used for critical speed and column loading calculations is 40 inches.

Determine End Fixity

The layout of the application shows that adequate space is available to use a double bearing EZZEMOUNT™ at each end. (See end fixity definitions on page 78-79 of our Precision Screw Assemblies Catalog). End Fixity = Type C.

Select a Screw Based on Critical Speed

Use previously determined values with the Critical Speed chart on page 85.

  • Max Travel Rate = 432 in/min
  • End Fixity = Type C
  • Length Between Bearing Supports = 40 inches

Based on the Critical Speed chart, the best choice, appears to be a 1000-0250 SRT. Since the lead of the 1000-0250 SRT ball screw is .250", the maximum rpm needed to achieve the maximum travel rate would be 1,728 rpm.

Check Column Strength of Screw

Use previously determined values in combination with the Column Strength Chart.

  • Load = 6.5 lbs
  • End Fixity = Type C
  • Length Between Bearing Supports = 40 inches

Based on the Column Strength Chart, the load is within the column strength of this screw.

NOTE: If this were a vertical application, the full 5,000 pound load would be used. Also, under high acceleration conditions, the inertia load must be determined and added to the total load for column considerations.

Step 3: Product Selection

Create a Reference Number for the Assembly

See page 93 for Reference Number System Chart.

The 1000-0250 SRT thread form is desired in a right hand thread. The end code used for machining this screw is end code 20. The type of end machining will be a Type 3 on both ends of the screw to allow for the mounting of double bearing EZZE-MOUNT™. One of the ends will have an extension to attach a coupling, the other will not. To determine the overall length of the assembly, add up the length of the ends plus the unsupported length:

  • One end Type 3K (drive end with keyway) = 4.03"
  • One end Type 3N (no drive end) = 2.71"
  • 40 inches between supports
  • Over-all length is 40" + 4.03"+ 2.71" = 46.74"

The Parts List Includes:

  • 1000-0250 SRT Ball Screw
  • Ball Nut Number: SBN10466
  • Flange Number: FLG7571
  • EZZE-MOUNT™: EZM-2020 - 2 required

To receive an assembly of these components with the EZZEMOUNT™, nut, and flange installed on the screw, the order reference number is:

1000-0250 SRT RH/EK/EN/46.74/SBN10466/FS Figure 1

 

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