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Ball Screw Assembly Capabilities

No standard ball screw can satisfy every application requirement. If you have any questions about the suitability of the current Beaver applications, don’t hesitate to call us. It may be possible to modify one of the Beaver assemblies using other material or finishes for your application. Beaver can work with you to design a custom device for your special requirements. Here are just a few of the unique devices with unusual requirements Beaver has designed and manufactured in the past:

  • Telescopic assemblies with a total retracted length smaller than extended length
  • Restricted envelope designs for mounting limitations and corrosion-resistant assemblies for environments that demand special material and finishes
  • Temperature-resistant designs utilizing special materials of construction and exotic lubricants for temperatures in excess of 300° F
  • Lightweight assemblies for aircraft and aerospace that take advantage of thin wall sections and high structural strength materials
  • Short stroke caged assemblies for limited travel applications that do not require recirculation of bearing balls
  • Special actuator configurations that accommodate gears, splines, and bearings as integral parts
  • Beaver manufactures ball bearing screws as small as 3/16 inch diameter, as large as 24-inch diameter -- as short as 1 inch in length, as long as 70 feet in length

Completed Ball Screw from Beaver Aerospace and Defense

 

Selection Procedures

SELECTION FACTORS

To select the right ballscrew for your application, determine the conditions under which it will operate and any special design factors which need to be considered. Operating conditions can include axial load, design life requirements, operating speed, length, and support bearing arrangements.

 

Design considerations such as lead, lead accuracy, system stiffness and ball nut selection must also be addressed.

 

When selecting ball bearing screws, factors like axial load, length, life requirements, support bearing arrangements, and speed are interrelated. Changing one factor will often require changing another. An example of the interrelationship of these factors is fine lead vs. high lead. A fine lead provides better positioning sensitivity and a lower drive torque, but it also results in higher rotary speed. A high lead means Lower rotary speed, but requires a higher drive torque, which may require a larger motor and related drive components. The chart below illustrates how basic changes in your selection will affect other factors.

OPERATING CONDITIONS

By applying four basic steps, you can select the appropriate size ballscrew to meet your operating conditions. These steps include:

  1. Determine the axial load.
  2. Determine the design life objective
  3. Verify safe operating speed.
  4. Verify safe compression load.

In addition to these four factors, you should also take into consideration any unusual environmental conditions, severe vibration or lubrication problems that might exist.

INCREASE IN

AFFECTS

HOW

Screw Length

Critical Speed

Compression Load

Decreases

Decreases

Screw Diameter

Critical Speed

Inertia

Compression Load

Increases

Increases

Increases

Lead

Drive Torque

Angular Velocity

Increases

Decreases

End Mounting Rigidity

Critical Speed

Compression Load

Increases

Increases

Load

Life

Decreases

Preload

Positioning Accuracy

System Stiffness

Drag Torque

Increases

Increases

Increases

 

1. DETERMINE THE AXIAL LOAD

The first step in selecting the right ballscrew is to determine the axial load as "seen by the screw." The method for calculating this load will vary depending on whether the ballscrew is being applied in a vertical or horizontal direction.

Vertical Applications -

In vertical applications, the axial load “seen by the screw” is the actual supported weight of the load.

 

Horizontal Applications -

In horizontal applications, the axial load “seen by the screw” is the actual supported weight of the load times the coefficient of friction of the load support bearings. For example, though it may be impossible to lift a 3000 lb. automobile, it is possible to push it. The load “seen” in pushing the automobile is the weight of the auto times the coefficient of friction of the wheels.

 

Imagine instead a 3000 lb. load supported on way bearings with a coefficient of friction of .2. The force required to move the load would be 3000 lb. x .2 = 600 lbs. This is the load as seen by the screw.

 

Typical coefficients of friction for various bearing surfaces are:

SLIDES/WAYS

BEARINGS

Bronze on steel — lubricated = .16

Ball Bushings = .001

Wood on steel — lubricated = .19 Rollerway Bearings = .005
Steel on steel — lubricated = .18 Ball Bearing Splines = .005
Steel on ceramic — lubricated = .11

Another practical way to determine the load in actual operation is to attach a spring scale to the load and pull it. Base the load on the moving force required, not the higher starting (breakaway) force.

2. DETERMINE THE DESIGN LIFE OBJECTIVE

The design life objective is the number of inches you require a ball bearing nut to travel during the life of the machine. To achieve this objective, you must select a ballscrew with a dynamic load rating sufficient enough to provide the required number of inches you want the ball nut to travel. The load/life data for current proven applications will assist you in selecting the approximate ballscrew size for your application.

Vertical Applications

In a vertical application, the weight of the load is always pushing down during the extend and retract strokes of the cycle. This means that this load is always applied in one direction and the contact angle of the bearing ball is always on the same side of the ball groove.

 

For example, if:

Length of Stroke: 8 inches
Cycle rate of machine: 25 strokes/hour
Estimated machine operation/day: 16 hours/day
Number of working days/year: 225
Number of years machine is designed for: 10 years

Counting one trip up (8 inches) and one trip down (8 inches), for each cycle, the design life in this example is: 8 x 2 x 25 x 16 x 225 x 10 = 14,400,000 inches. Note that the stroke length has been multiplied by 2 since the load is always on the same side of the ball groove during both extend and retract strokes.

 

Horizontal Applications

In a horizontal application, the axial load is applied in both directions, shifting the bearing balls contact angle back and forth to each side of the ball groove. Therefore, it is not necessary to multiply the stroke length by 2 as was required in the vertical application.

 

Using the same example presented for the vertical application, the calculation to determine the design life objective would instead be: 8 x 25 x 16 x 225 x 10 = 7,200,000 inches.

 

Once the axial load and the design life have been determined, you can use this load/life relationship to help you select the right ballscrew model. Since the load/life relationships are analogous to the B10 rating common in the ball bearing industry, it is easy to predict the life of a ballscrew under various loads. The usable life of a ball bearing screw, or its rated load, is measured at the length of travel where 90% of a group of ballscrews will complete or exceed before the first evidence of fatigue develops.

3. VERIFY THE CRITICAL SPEED

Once you have made your selection, you must first verify that the planned operating speed is within the safe limits of that model. The three factors that determine the safe speed of a ball bearing screw are the screw diameter, the screw length and the rigidity of the end mountings (end fixity).

 

The critical speed formula will help you ensure that the model you have selected will operate safely within your speed requirements.

4. VERIFY SAFE COMPRESSION LOADING

In addition to critical speed, you must also verify safe compression load. If a sufficiently heavy load is applied to a long ball bearing screw it could buckle. The three factors that determine a safe compression load are: the length between the load point and the end bearings, the load, and the rigidity of the end mountings (end fixity).The compression load formula on page 20 will allow you to verify that your selection meets safe compression load requirements.


MOUNTING REQUIREMENTS

When selecting a ball bearing screw, a number of load conditions must be considered. Excessive compression and tension loads should be avoided. Radial and Eccentric loads can greatly reduce a ball bearing screw’s rated life as well. Click here to view Mounting Requirements.


DESIGN CONSIDERATIONS

In addition to the operating conditions that may be associated with your application, a number of design considerations must also be addressed in order to select the correct ballscrew assembly.

1. LEAD

You must determine what lead will be most appropriate for your application. The lead is the distance the nut will travel with one revolution of the ballscrew.

 

Smaller leads will make the nut travel slower over the length of the screw for a given RPM while a larger lead will allow the nut to travel faster over the length of the screw at the same RPM. The most commonly utilized inch leads are .200, .250, .333, .500, 1.000 and 2.000. The most common metric leads are 5mm and 10mm.

2. LEAD ACCURACY

Once you’ve selected the optimal lead, determine the necessary lead accuracy you will require for your positioning needs. The lead accuracy of a ballscrew is a measure of the cumulative lead error per foot. .0005”/foot cumulative (ISO class 3). Units can also be supplied to ISO classes 1, 5, 7 and 9.

3. BALL NUT SELECTION

In selecting a ball nut, several design factors should be considered - including mounting flexibility, design envelope, load carrying capability and backlash considerations.

  • Style of Nut
    The style of the nut you select will depend on your particular mounting and design requirements. Beaver offers a variety of ball nut designs, each intended to address specific requirements. For example, you may wish to select the “V” thread style because of the mounting flexibility it offers. If you need the convenience of an integral flange, mounting trunions, trunion holes, gimbals, splines or integral gear, these options are available with external or internal ball returns.
  • Preload/Non-Preload
    Ball bearing screws possess a certain amount of inherent axial freedom and play commonly referred to as backlash. If the load is always maintained in one direction, backlash is not an application criterion because the balls will always be held to one side of the ball thread groove. However, where a reversing load is present, coupled with positioning accuracy requirements, a preloaded ball nut is necessary to remove the backlash while also providing an increase in stiffness. While each ball nut type is available in a non-preload version, you may also select preloaded ball nuts for most models. The two preload methods available include a double nut preload and an integral preload.

4. DETERMINE IF SYSTEM STIFFNESS IS DESIRABLE

On some applications, most notably those on CNC and NC machine tools, a high degree of system stiffness is desirable because positioning error must be held to an absolute minimum. Such cases necessitate the use of a preloaded ball nut. A ball bearing screw assembly with a high spring rate (stiffness) resists deflection when load is applied, thereby minimizing positioning error. Spring Rate formulas to help you select the model that provides the required spring rate for your application are found here.

5. LENGTH

In determining ballscrew length, three parameters must be considered. These are the overall length from one end of the assembly to the other, the ball thread length and the stroke travel length. The threaded length of the ballscrew assembly equals the stroke length plus the ball nut length plus any additional threaded length you may desire at each end of the stroke.

6. OPTIONS

Depending on the type of unit you select you may want to specify additional optional components such as preloaded nuts, mounting flanges, housings, over stops and end journal machining.

 

MOST FREQUENTLY ASKED QUESTIONS ABOUT BALL BEARING SCREWS

 

 

 

 

 

 

 


 

Beaver Aerospace & Defense, Inc., is a subsidiary of Phillips Service Industries, Livonia, Michigan

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© Beaver Aerospace & Defense, Inc. - 11850 Mayfield.  Livonia, MI. 48150 
Ph: 734.853.5003