Manual Lathe Application

Application Description

Given the following requirements, select an acme screw for an application which uses precision Acme screw systems for an automatic part feeder on a machine.

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 +/- ¼"

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 × N (μ = Coefficient of Friction of the guidance system)

Using Nook linear bearings in this application we can determine:

  • μ = Coefficient of Friction for lubricated Nook Linear Bearings = .0013
  • N = Load = 5000 pounds
  • F = μ × N
  • F = .0013 × 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. (See Image 1 in Figure 1 Below)

  • V peak = 2 × V avg.
  • V avg. = 3.6 in/second or 216 in/minute
  • V peak = 2 × 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 inches, but extra screw length should be considered for travel nut, carriage, and or 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 an extra 4 inches 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 EZZE MOUNT™ at each end (see end fixity definitions on page 12 of our Precision Screw Assemblies Catalog). End Fixity = Type C.

Select Acme Screw Systems Based on Critical Speed

Use previously determined values in combination with the Critical Speed Chart on page 17 of our Precision Screw Systems Catalog.

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

Based on the Critical Speed Chart, a 1"- 5 Acme Screw (1 inch diameter, 5 threads per inch) is selected.

Check Column Strength of Screw

Use previously determined values in combination with the Column Strength Chart on page 16 of our Precision Screw Systems Catalog.

  • Load = 6.4 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.

Check the PV Value

This relates the pressure load to the speed of the nut. First find the actual P value based on the calculation. Using the formulas from page 11 of the Precision Screw Catalog:

P = Actual Operating Load × 1250 psi
Nut Dynamic Load Capacity
6.5 lbs = 3.2 psi
2,500 lbs

Next the "V" value or maximum relative speed between the screw and nut is:

V (ft/min) = Outside Dia. of Screw (in) × π × Operating Speed (rpm) = 3.2 psi
12" / ft
1" × π × 2160 rpm = 565 ft / minute
12" / ft

This results in a PV value of 3.2 times 565 or 1,808 below the maximum recommended value of 2,700.

Step 3: Product Selection

Create a Reference Number for the Assembly

See page 31 of our Precision Screw Catalog for Reference Number System Chart. The 1"- 5 Acme Screw is thread form code 105. The screw material is right-hand thread, alloy steel. The end code used for machining this screw is end code 17. The type of machining will be a Type 3 on both ends of screw to allow for mounting a double bearing.

EZZE-MOUNT™. One end will have a section 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) = 3.65"
  • One end Type 3N (no drive end) = 2.33"
  • 40 inches between supports
  • Over-all length is 40" + 3.65"+ 2.33" = 45.98"

The Parts List Includes:

  • One Plastic Acme Nut -30105
  • One Steel Flange - 70275
  • EZZE-MOUNT™ Bearing blocks (2 req'd) - EZM-3017

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

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.

105 - RA / EK / EN / 45.98 / 30105 / FS

NOTE: The nut will be installed with the flange facing toward the first specified end. In this example, the EK end.

Figure 1

 

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