Worm Gear Essentials – Column Strength

We’re looking at the elements that help a designer determine the right worm gear screw for an application, based off the excellent white paper “How to Size a Worm Gear Jack” from Ron Giovannone, Director of Application Engineering and Business Operations with Nook Industries in Cleveland, Ohio.

In this post, we look at how Column Strength factors into design considerations.

Column strength is the ability of the lift shaft to hold compressive loads without buckling. A compression load is a load that tends to squeeze the screw axially, which can cause buckling. With longer screw lengths, the column strength of the lift shaft may be substantially lower than nominal jack capacity.

In order to determine the compressibility of a given travel length, you must first determine your mounting condition.

A simplified formula to calculate the column strength in pounds is as follows:

WHERE:

Pcr          =             Maximum Load (lb)

d             =             Root Diameter of Screw (inch)

L              =             Distance between nut and load carrying bearing (inch)

Fc            =             End Fixity Factor

0.25 for mounting condition A

1.00 for mounting condition B

2.00 for mounting condition C

4.00 for mounting condition D

The above formula can only apply when the slenderness ratio (the length divided by the radius of gyration) is not exceeded.

Note: If you can ensure that the load will always be held in tension, you don’t need to consider column loading.

Related tools:

Inch Column Strength Calculator

Metric Column Strength Calculator

Worm Gear Essentials – Tonnage

This week, we continue our review of the white paper “How to Size a Worm Gear Screw Jack” by looking at the role Tonnage under load can affect the sizing of the linear motion system.

The load capacity of a jack is also limited by the physical constraints of its components, such as its drive sleeve, lift shaft or bearings. All anticipated loads should be within the rated capacity of the jack. Loads on the jack in most applications include: static loads, dynamic (or moving) loads, cutting forces or other reaction forces and acceleration/deceleration loads.

For shock loads, the peak load must not exceed the rated capacity of the jack, and an appropriate design factor should be applied that is commensurate with the severity of the shock.

For accidental overloads not anticipated in the design of the system, jacks produced by Nook Industries can sustain the following overload conditions without damage: 10 percent for dynamic loads, 30 percent for static loads.

Jack Application Data Form

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Worm Gear Essentials – Horsepower

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

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