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Worm Gear Essentials – Horsepower

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The following is taken from an excellent white paper from Ron Givannone, Director of Application Engineering and Business Operations at Nook Industries (hyperlink). Titled “How to Size a Worm Gear Jack,” it looks at the key factors in figuring out what size and configuration of jack will work based on the needs of your application. Over the next few weeks, we’ll look at the different factors, and offer a bit more background.

This week, how Horsepower limitations can affect jack sizing.

When determining the lifting power of a jack, it’s a common mistake to assume the lifting capabilities of a jack are determined solely by its tonnage size. The load’s capacity is more often determined by its horsepower limitations. For example, a 10-ton jack may only be able to lift a one-ton load, because it is temperature-limited by the working horsepower it requires to lift the load.

The horsepower limit of the jack is a result of its ability to dissipate the heat generated from the inefficiencies of its components. The maximum horsepower value represents the point at which the heat that is generated by the working horsepower to move a given load meets the maximum temperature of the internal components. The working horsepower to move a given load is calculated by using the following formula:

How well a jack can dissipate heat is 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 gear must not exceed 200 degrees Fahrenheit.

Looking for help in figuring out the right horsepower? Here are some tools:

Calculate Horsepower with this calculator

Calculate Kilowatts

Worm gear jacks definitions and technical data

Acme and Lead Screw Products, Ball Screw Jacks, Ball Screw Products, Linear Motion Design Considerations, Planetary Roller Screw Products, Screw Accessories, Screw Jacks, Worm Gear Ball Screw Jacks, Worm Gear Machine Screw Jacks

Three Popular Screw Types Defined

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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:

Acme

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:

ball

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

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.

Ball Screw Jacks, Linear Motion Design Considerations, Screw Jacks, Worm Gear Ball Screw Jacks, Worm Gear Machine Screw Jacks

8 Design Considerations for Worm Gear Jacks

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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.