Category Archives: Screw Jacks

Linear Motion 101 – Learn From the Pros

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Ball Screw Jack vs. Acme Screw Jack

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Tips for how to select the right one for your application
Worm Gear Screws Jacks can provide long duty life, high load capacity and flexible design. They come in two major categories, Ball Screw and Machine Screw. In this post, we hope to help you identify the best type for your application.

Ball Screw Jacks use a ball screw and nut made from hardened alloy steel with bearing balls carrying the load between nut & screw. This rolling action reduces the friction between nut and screw, permitting smooth and efficient load movement that requires approximately 1/3 less torque than a machine screw jack with the same load.

inch ball

Machine Screw Jacks incorporate an alloy or sometimes stainless steel worm which drives a high strength bronze worm gear, or drive sleeve. The worm shaft is supported on anti-friction tapered roller bearings with external seals that prevent lubrication loss. The drive sleeve can also be supported on tapered roller bearings, or ball thrust bearings. Rotation of the drive sleeve causes the acme thread lifting screw to translate or rotate, depending on the jack configuration.

inch inverted machine

Because of their efficiency and lower power requirements, Ball Screw Jacks are often preferred. However, several factors can make Machine Screw Jacks preferable. For quick reference …

Machine Screw Jacks are best used for:
• Resistance to backdriving
• Environments with vibration
• Manual operation
• High static loads
• Corrosion resistance (with stainless steel versions)

Ball Screw Jacks are preferred for:
• Long travel lengths
• Long, predictable life
• High duty cycles
• Oscillating motion

Both types can be metric or inch, come in several types (Upright, Inverted, Upright Rotating and Inverted Rotating) and multiple jacks can be laid out in H, U, T and In-Line arrangement.

You can also employ multiple jacks in tandem, depending on the physical design and size of the equipment, its stiffness and the guide system. This will, however, introduce challenges with drive, alignment and synchronization.

Any jack system is limited by multiple constraints: load capacity, duty cycle, horsepower, column strength, critical speed, type of guidance, brake motor size and ball screw life. To properly size your jack for these constraints, application information must be collected.

All Your Equations In One Place


When a linear motion solution is in place & running, it’s easy for the casual observer to think it looks easy. That’s what our industry is all about; making the difficult or even impossible look easy.

But what few people outside the industry realize is just how much work needs to go into the design of a successful linear motion system. The old adage “Measure twice, cut once” doesn’t even begin to cover all the variables, that have to be dealt with.

Now, a new app looks to make that successful design at least a little easier. The Design Guide Pro not only offers selectors for Bevel Gears, Worm Gear Jacks, Bearings and Electric Cylinders, but it also has a calculator section. Here, you’ll find tools to help you establish Energy, Critical Speed, Column Load, Torque and Nut Life as well as helping with Unit Conversions.

All in the palm of your hand. Check it out today by clicking here.


Three Popular Screw Types Defined

When considering the vast majority of applications in which machine screws are used, it’s important to review the functions of some of the major types of screws. Below, we’ll take a look at the designs, functions and more while we define acme, ball and planetary screws.

Acme Screws:


The acme screw thread, sometimes referred to as the trapezoidal thread, is used for lead screws. They are often needed for large loads, or when the environment is less than desirable.

The acme thread form has been around for over a century, replacing square thread screws which had straight-sided flanks and were difficult to manufacture.

There are two main classes of acme thread forms: general purpose (G) and centralizing (C). The general purpose and centralizing thread forms have a nominal depth of thread of 0.50 x pitch and have a 29 degree included thread angle, which has allowed companies to develop unique screw diameters and leads. European metric Trapezoidal thread forms have a 30 degree Included thread angle.

When compared to general-purpose thread forms, centralizing threads are manufactured with tighter tolerances and reduced clearance on the major diameter. For instance; If an acme nut is side loaded with a radial load, a “G” class will wedge when the nut thread flanks come in contact with the screw thread flanks. To prevent this wedging, a “C” class thread form can be used, since it utilizes less clearance and tighter tolerances are allowed between the major diameter of the nut and the major diameter of the screw.

Industry leaders have developed several unique thread forms, such as stub acme forms and 40 degree included angle, which allow them to provide a variety of diameter and lead combinations.

Ball Screws:


For loads requiring a greater amount of efficiency, companies often turn to ball screws. A ball screw assembly is a device comprised of a nut, screw, and reciprocating ball bearings. The bearings provide the thread engagement between the nut and screw.

Ball screws offer an efficient means for converting rotary motion to linear motion. A ball screw is an improvement over an acme screw just as an anti-friction ball bearing is an improvement over a plain bushing.

In the long run, ball screw systems can prove to be a cost-effective alternative to pneumatic or hydraulic systems, which require constant electrical and air power.

Planetary Roller Screws:


Planetary roller screws are remarkable devices designed to convert rotary motion into axial force or vice versa.

The planetary roller screw design offers multiple advantages and reliability for the most demanding applications when compared with other lead screw types due to its rolling motion. These screws offer high efficiency even in relatively shallow lead designs.

The multitude of contact points can carry large loads and provide very high resolution (small axial movement) when using very shallow leads. Planetary roller screws produce high rotational speeds with faster acceleration without adverse effects.

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.