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8 Design Considerations for Worm Gear Jacks

8 Factors You Need to Consider
No matter the type of worm gear jack, machine or ball, there are 8 factors that need to be known and addressed in the design of a solution. In this post, we’ll start looking at these design constraints and how they can determine the sizing, placement and configuration of your worm gear jack screw.

Stainless machine upright1. Load Capacity
The load capacity of the jack is limited by the physical constraints of the components (drive sleeve, lift shaft, bearings, etc.). All types of anticipated loads must be calculated, and be within the rated capacity of the jack. These loads can include: static, dynamic, moving, acceleration/deceleration loads as well as cutting and other reaction forces.

Appropriate design should also be made for shock loads, and should not exceed the rated capacity of the jack.

To accommodate accidental overloads, jacks can sustain the following overload conditions without damage – 10% for dynamic loads, 30% for static.

2. Duty Cycle
Duty cycle is the percentage of time on as opposed to total time. Recommended duty cycles for the two styles of jacks at max horsepower are:
• Ball screw jacks 35% (65% off)
• Machine screw jacks 25% (75% off)

The largest determining factor in calculating duty cycle is the ability of the jack to dissipate heat that builds up during operation. Anything that reduces or increases the generated heat increases or decreases duty cycle accordingly. Additionally, jacks may be limited by their maximum operating temperature (200°F) and not duty cycle.

metric inverted3. Horsepower Ratings
Horsepower values are influenced by many application-specific variables including mounting, environment, duty cycle and lubrication. The best way to determine whether performance is within horsepower limits is to measure the jack temperature. The temperature of the housing near the worm must not exceed 200°F.

The horsepower limit of a jack is a result of the ability to dissipate the heat generated from the inefficiencies of its components, based on intermittent operation. Special consideration should be given for multiple jack arrangements, as total horsepower required depends on horsepower per jack, number of jacks, the efficiency of the gear box or boxes and the efficiency of the arrangement.

If needed horsepower exceeds the maximum for the jack selected, several solutions are possible:
Use a larger jack
• If it is a Machine Screw Jack, look at a comparable Ball Screw Jack
• Operate at a lower input speed
• Use a right angle reducer

inch inverted machine4. Column Strength
Column Strength is the ability of the lift shaft to hold compressive loads without buckling. With longer screw lengths, column strength can be substantially lower than nominal jack capacity.

If the lift shaft is in tension only, the screw jack travel is limited by the available screw material or by the critical speed of the screw. If there is any possibility for the lift shaft to go into compression, the application should be sized for sufficient column strength. Designers should also be aware of effects of side loading. Jacks operating horizontally with long lift shafts can experience bending from the weight of the screw.

If column strength is exceeded, there are several options:
• Change the jack configuration in order to put the shaft in tension
• Increase jack size
• Add a bearing mount for rotating jacks
• Change the lift shaft mounting condition, for example, from clevis to top plate

5. Critical Speed
The speed that excites the natural frequency of the screw is referred to as the critical speed. The critical speed will vary with the diameter, unsupported length, end fixity and rpm of the screw.

Because of the nature of most screw jack applications, critical speed is often overlooked. However, with longer travels, critical speed should be a major factor in determining the appropriate size jack. Since critical speed can also be affected by the shaft straightness and assembly alignment, it is recommended that the maximum speed be limited to 80% of the calculated critical speed.

inch ball6. Type of Guidance
All linear motion systems require both thrust & guidance. Worm gear jacks are designed to provide thrust only and a guidance system should be designed to absorb all loads other than thrust. Preferred systems include hardened ground round shafting or square profile rail.

7. Brakemotor Sizing
To ensure safety, a brakemotor is recommended for worm gear jack screws where there is the possibility of injury. Horsepower requirements will determine the size of the motor, and once selected, verify that the standard brake has sufficient torque to both stop and hold the load.

Lastly, high lead ball screws may require larger, nonstandard brakes to stop the load, to ensure against excessive “drift” when stopping.

8. Ball Screw Life
A major benefit of the use of ball screw jacks is the ability to predict the theoretical life of the ball screw, and all major manufacturers will provide life charts for their products.

Once these factors are understood and accounted for, and paired with the features and benefits of Machine and Ball Screw Jacks, selecting the right one for your application should be considerably easier.

Why Use Worm Gear Screw Jacks with In-Line Arrangement

For worm gear screw jacks in the steel tube industry, in-line proves to be the best choice for arrangement in getting the job done quickly and efficiently.

in line arrangementTo give you an idea of how an in-line arrangement works in a company’s favor, here’s a real-life scenario to observe: A steel tube manufacturer is developing a new OD polisher that will increase production by 22 percent. Because of the increased production time, the set-up crew is unable to set the feed table manually and is looking to automate the feed table height using screw jack actuators.

The feed table length is 24 feet and weighs 5,600 pounds with the largest diameter steel pipe. The table height will need to change approximately once every 15 minutes, but no more than 10 times a day. Maximum height change is nine inches. The rate is .4 inches per second.

By our specifications, the in-line arrangement comes with a single three-HP AC Motor with 1750 RPM and drive, and it comes with the possibility to be removed and driven by hand. With hand-driven possibilities, a machine screw jack with a 24-to-1 gear ratio is needed to prevent back driving.

The mounting constraints call for an upright translating jack with a clevis rod end. Due to the length of the feed table, four jacks will be used in line with a center mounted motor through a single gearbox.

Other specifications for the in-line arrangement include a compression load, a total travel of 14 inches and the ability to move .25 inches in one second.

Get to Know Miniature Profile Ball Rail Lubrication Systems

mini profile ball rail

When dealing with miniature ball rail systems, having an effective lubrication system is essential for keeping the systems clean and running smooth. Miniature profile ball rail lubrication reduces friction and wear, prevents corrosion, dissipates heat and increases overall service life.

There are plenty of benefits to using industry-leading lubrication systems for miniature ball rails. Low-friction end seals effectively restrict dust while stainless steel endcaps act as scrapers, allowing for easy maintenance. Ball recirculation design, with hole and channel constructs fully sealed by a plastic frame and endcaps, reduces contact surface between steel ball and metal. This minimizes noise and adds to lubrication efficiency, reducing preventative maintenance.

A thin layer of oil separates the rolling elements from the raceway at the contact zone of the stainless steel ball rail. Lubrication systems allow the ball rails to move smoothly during short stroke movement. This lubrication assists in ensuring high-speed running production.

We recommend using LBL1, a lubricant formulated for rolling friction, with linear bearings. This lubrication is made to protect highly polished bearings from surface wear and corrosion.

Certain factors affect the reapplication levels, including speed, load, stroke length and operating environment. Practical observation is the only sure way to find a safe lubrication interval.

We recommend synthetic-oil based lithium with a viscosity between ISO VG32-100 soap grease if lubricant grease is required.

Ball Screw Solutions for Long Travel & High Speed

Design World’s always excellent Danielle Collins posted a great article about how Design Engineers can achieve both high speed and long travel with Ball Screws.

Didn’t know you could do that, did you?

Traditional solutions involve Belt Drives, Rack and Pinion Systems or Linear Motors. But each of those come with their own set of drawbacks. Ball Screws are an ideal solution for applications that are sensitive to thrust force or positioning accuracy

Yet Ball Screws are rarely looked at for high speed/long travel applications. Why?

Critical Speed

Any long cylindrical object will naturally sag in the middle. Add rotation, and you will get a whip effect, similar to a jump rope. The speed of rotation where that effect starts is the Critical Speed.

Obviously, one way to limit this effect would be to have a shorter unsupported span. But how do you limit the effect if your application calls for a long travel span?

The Solution – Ball Screw Supports

ball screw supportsSome customers have made their own ball screw supports, usually paired on either side of the ball nut in pairs of 2, 4 or 6, to reduce the unsupported distance, essentially quadrupling the critical speed for each pair used. Depending on the application, they can even allow you to select a smaller diameter ball screw, without compromising performance.

So, have you used this solution? Or do you have an application where it might be a suitable alternative?

14 Key Terms in Understanding Electric Cylinders

Electric Cylinders are commonly used in satellite dishes, solar panels and other large industrial environments. When dealing with Electric Cylinders, these 14 terms are essential to know:

1. BACKLASH

Backlash can sometimes be beneficial. While backlash can create inaccuracies, components that can tolerate a small amount of backlash can create space for contaminants to pass through the component. Backlash in cylinders occurs wherever reversible load conditions exist. Backlash is less than .015 inches for all but the largest cylinder models. Ball Screw Cylinders can be factory adjusted to reduce backlash at the lift shaft by selecting bearing ball size in the ball nut. This selective fit technique can be used to achieve a minimal lash between the ball nut and ball screw of .003 inches to .005 inches. Precision ball screws with preloaded nuts can be supplied when less than .003 inches of backlash is required.

Electric Cylander Arrangement 12. REACTION TORQUE

When an electric cylinder is used to move a load, the actuator tube must be secured to prevent rotation. The reaction torque required to prevent rotation is a function of the screw lead and the load applied on the cylinder.  Prior to installation, the actuator tube can rotate freely in or out of the cylinder without movement of the input worm. This ability to rotate aids installation but prevents the optional rotary limit switch from being factory preset for end of travel positions. Rod-Type Limit Switches prevent tubes from freely rotating but are not intended to absorb the rod reaction torque.

3. TRAVEL LENGTH

Electric Cylinders are not pre-assembled or stocked with standard length screws. Each cylinder is made to order based on travel length. Cylinders can be built with non-standard lead screws to change the cylinder’s operating speed, or  if required, ground or preloaded screws.

4. LEAD ACCURACY

Lead accuracy is the difference between the actual distance traveled and the theoretical distance traveled based on lead. For example: Consider a lift shaft with a .5-inch lead and ± .004-inch/ foot lead accuracy. If the shaft is rotated 24 times, the distance the nut moves is 11.996 to 12.004 inches. The rolled thread screws, as employed in products, are held within ± .004-inch-per-foot lead error.

5. INPUT TORQUE

The input torque is the rotary force required at the input of the cylinder to generate an output force at the actuator tube. This number, multiplied by the load, is the required input torque.

6. INPUT SPEED

DD and RAD Electric Cylinder models are rated at 1,725 rpm input. If provided with a servo motor, cylinders may be operated up to 3,000 rpm provided horsepower and temperature ratings are not exceeded.

NOTE: Maximum horsepower values should not be exceeded.

7. DUTY CYCLE

Duty cycle is the ratio of run time to total cycle time. Some of the electrical energy input to an electric cylinder is converted into heat. The duty cycle is limited by the ability of the electric cylinder to dissipate this heat. An increase in temperature can affect the properties of some components resulting in accelerated wear, damage and possible unexpected failure.

Electric Cylander Arrangement 28. SELF-LOCKING AND BREAKS

Self-locking occurs when system efficiencies are low enough that the force on the actuator lifting tube cannot cause the drive system to reverse direction. Electric Cylinders that utilize acme screws and have ratios of 20-to-1 or greater are self-locking and, in the absence of vibration, hold loads without backdriving. All other models require a motor brake to prevent backdriving.

10. END-OF-TRAVEL STOPS

Travel stops are not standard. A limit switch and a brake should be used to stop the motor. Mechanical stops can cause damage to the cylinders because most electric motors will deliver stall torques much higher than their rated torques and motor inertia can cause severe shock loads. For hand operation, mechanical stops can be provided.

11. MAXIMUM LOAD

The maximum load is the thrust load, including shock, that can be applied to the actuator without damaging the assembly.

12. DYNAMIC CAPACITY

The dynamic capacity is the maximum allowable thrust load based on horsepower, thrust bearing and screw limitation. The dynamic capacity can determine the life of a given material.

13. TENSION LOAD

A tension load is the load that tends to stretch the screw. Good materials should be able to retain stiffness and withstand stretching.

14. COMPRESSION LOAD

A compression load is the load that tends to squeeze the screw. Good materials should be able to withstand buckling and maintain straight columns.