All posts by admin

The New Modular Actuator Calculator Provides Solutions for Bearing Life & Load Considerations

Engineering_Calculator_ThumbnailWhen trying to figure out the load size for an actuator, many engineers find themselves making guesses for the bearing considerations. This is because the bearings are buried within the actuator, which makes it impossible to determine their exact location.

Well, thanks to a new modular actuator calculator, engineers can stop guessing and receive precise load figures. The German-developed calculator works by simply choosing the desired product, entering its load definitions and allowing the tool to calculate the bearing stress and load to determine the bearing life of the product. The results can be viewed in an easy-to-read, printable format.

Now, if you want a particular product to last five years, all you have to do is plug in a few definitions and the calculator will determine the load necessary to achieve that life expectancy.

The free-to-use calculator is compatible with Safari and Internet Explorer. To access this resource, visit here.

Advantages of a U-Shaped Worm Gear Jack Screw Arrangement

In worm gear screw jacks, the “U” arrangement’s configuration is often preferred for manufacturers in the food industry.

U ArrangementFor example, a leading cookie manufacturer could be adding a new product that requires a greater distance to the top heating element of the conveyor oven. The oven originally only had a static-top heating element and with this new order, it needs to be adjustable up to 14 inches. The top heating element weighs 5,000 pounds. The manufacturer anticipates only making adjustments to the height once or twice a month.

Our specifications for the U arrangement include a single 5-Horsepower 1750 AC Motor that allows full travel in 36 seconds, food-grade grease, compression load and a double safety factor. The actuators and power train must be located outside of the oven frame. Travel rate is negligible as long as the total travel can be reached in less than 60 seconds. The application’s infrequent cycles makes the use of a machine screw jack the best fit.

Upright rotating jacks allow the jacks to be easily retrofitted to the existing oven with minimal modifications. The jacks will be fully loaded in both the retracted and extended position, but because of the retrofit condition, having a support bearing on the lift shaft will not be possible. A 10-ton jack was selected for the mounting condition to fulfill safety requirement.

For calculating column strength, check out Nook’s calculator, as well as other useful engineering calculators here.

H Arrangement of Worm Gear Screw Jacks

One of the most common arrangements used for worm gear screw jacks is the “H” arrangement. The name comes from the arrangement’s shape, which is made up of four jacks, three gearboxes and a 3 HP AC Motor capable of a dual-speed 1750/800.

H ArrangmentAn example of this arrangement could be a manufacturer of steel frames for the commercial dairy industry is building a material lift which contains a stack of prefabricated frames. The material lift will index up as each frame is removed by an automated grip from the top of the stack. The jack will index up 1 inch in 2 seconds every 30 seconds. After the last frame is removed, the jacks will fully retract to the collapsed position in 6 seconds waiting for the next load of frames. Complete cycle time is 10 minutes running 6 hours per day, 5 days a week. The design calls for a four-jack arrangement lifting from underneath the lifting stage, driven by a single motor.

The frequency of these cycles and suggested design life makes the use of a ball screw jack the best option for this application. Using upright translating jacks allows the jacks to be located under the material lift without creating obstruction.

When fully loaded, the frames hold a total weight of 16,800 pounds, but when the load is fully extended, the weight is less than 5,000 pounds. The compression load travels 6 inches. The desired design life for the arrangement is one year, as the application is expected to go through 3,120 cycles a year.

 

Worm Screw Jack T Arrangement Advantages

With all the arrangements available, what is it about the “T” arrangement that appeals to a manufacturer?

T ArrangementWell, for a manufacturer looking to periodically raise and lower a cylindrical mixer 8 inches during the mixing cycle to allow product testing, the T arrangement ensures the load is lifted uniformly and safely.

The 1,700-pound cylinder is mounted on a movable cart, allowing for the final product to be moved to a dispensing station. The arrangement offers a redundancy and reliability that gives the manufacturer assurance that their production will not be compromised.

For this configuration, a ball screw jack will be used to minimize the size of the motor drive. Based on the mounting frame, the inverted rotating configurations will be used. The jacks will be loaded in tension, therefore column strength will not need to be considered.

Specifications for the arrangement include: a single 1750 AC Motor and 2.5HL-BSJ with a 12–to-one gear ratio and the capability to travel 8 inches in five seconds.

Helpful Terms and Tips for Understanding Precision Ball Splines

Ball Splines are convenient and efficient devices that allow friction-free linear motion while transmitting torque. Because of their reliability and high efficiency, they are utilized to replace conventional splines. In a ball spline assembly, recirculating bearing balls carry the load between the rotating member (inner race) and the rotating/translating member (outer race). These defined terms, processes and applications will give you a better idea of how and why ball splines matter in the precision engineering industry.

10 BALL SPLINE TERMS:

1. ACTIVE CIRCUITS

The closed path that the bearing balls follow through the outer race is referred to as a circuit. The number of potential circuits varies with the diameter of the spline shaft. When a circuit is loaded with bearing balls, it is referred to as an “active circuit.”

2. RETURN GUIDES

The outer race component through which the bearing balls are recirculated is referred to as the return guide.

3. BALL CIRCLE DIAMETER

The ball circle diameter is the diameter of the circle generated by the center of the bearing balls when in contact with the inner and outer race.

4. LAND DIAMETER

The land diameter is the outside diameter of the inner race. This diameter is less than the ball circle diameter.

5. ROOT DIAMETER

The root diameter is the diameter of the inner race measured at the bottom of the groove. This is the diameter used for critical speed calculations.

6. STRAIGHTNESS

Internal stresses may cause the material to bend. When ordering random lengths or cut material without end machining, straightening is recommended. Handling or machining of splines can also cause the material to bend. Before, during and after machining, additional straightening may be required.

7. LIFE

A ball spline assembly uses rolling elements to carry a load similar to an anti-friction (ball) bearing. These elements do not wear when properly lubricated during normal use. Therefore, ball spline life is predictable and is determined by calculating the fatigue failure of the components.

8. FRICTION

The use of rolling elements in a ball spline results in a low coefficient of friction.

9. ROTATIONAL LASH

Backlash or lash is the relative rotational movement of an outer race with no rotation of the inner race (or vice versa).

10. SELECTIVE FIT

When less than standard lash is required and a pre-loaded outer race cannot be used, outer races can be custom fit to a specific inner race with bearing balls selected to minimize rotational (angular) lash.

 

FIVE  LOAD DEFINITIONS:

1. DYNAMIC TORQUE LOAD

The dynamic torque load is the torque load which, when applied to the ball spline assembly, will allow a minimum life of 1,000,000 inches of travel.

2. STATIC TORQUE LOAD

This is the maximum torque load (including shock) that can be applied to the spline assembly without damaging the assembly.

3. OVERTURNING LOAD

A load that rotates the outer race around the longitudinal axis of the inner race is an overturning load.

4. SIDE LOAD

This is a load that is applied radially to the outer race.  Although a side load will not prevent the ball spline from operating, the outer race is not designed to operate with a side load, such as those generated from pulleys, drive belts or misalignment.

5. PRELOAD

Preload is a load introduced between an outer race and screw assembly that eliminates radial movement. Preloaded assemblies provide zero backlash for excellent repeatability and increased system stiffness.

 

OPTIONAL STANDARD KEYWAYS

Typically, outer races are mounted by machining a keyway into the outer race, inserting a key, and then sliding the outer race into a keyed bore. Standard machined keyways are available.

 

TRANSFERRING OUTER RACES FROM SHIPPING ARBOR:

STANDARD RACES

Ball spline outer races are shipped on arbors. Transferring the outer race from the arbor to the ball spline can be achieved by placing the arbor against the end of the spline and carefully sliding the outer race onto the inner race.

If the I.D. of the arbor is not able to slip over the O.D. of the end journal, apply tape to the journal to bring the O.D. up to the root diameter. The outer race can then be transferred across the taped journal onto the ball spline. Removal of the arbor from the outer race will result in the loss of the bearing balls. The set screw is used for transportation only and needs to be completely removed after installation.

 

LUBRICATION

Proper and frequent lubrication must be provided to achieve predicted service life. A 90% reduction in the ball spline life should be anticipated when operating without lubricants.

Standard lubrication practices for antifriction bearings should be followed when lubricating ball splines. A light oil or grease (lithium-based) is suitable for most applications. Lubricants containing solid additives such as molydisulfide or graphite should not be used.

Lubrication intervals are determined by the application. It is required that spline assemblies are lubricated often enough to maintain a film of lubricant on the inner race.

Ball screw lubricant protects against inter-ball friction, wear, corrosion and oxidation.  We recommend E-900, a lubricant that will provide a lasting film for wear protection and resistance to corrosion. With an operating range of -65° to +375°F, E-900 has low rolling friction characteristics and helps reduce inter-ball friction in ball spline assemblies. For optimum results, the ball spline assembly should be in good repair and free of dirt and grease. Used regularly, lubrication will extend the life of ball spline assemblies. It should be applied generously on the entire length of the spline.

 

TEMPERATURE:

We use ball splines which will operate between -65°F and 300°F with proper lubrication.

END MACHINING:

Annealed ends can be provided on precision ball splines to facilitate end machining of journals.

END FIXITY:

End fixity refers to the method by which the ends of the spline are supported.

CRITICAL SPEED:

The speed that excites the natural frequency of the spline inner race is referred to as the critical speed. Resonance at the natural frequency of the inner race will occur regardless of orientation (vertical, horizontal, etc.).

The critical speed will vary with the diameter, unsupported length, end fixity and rpm. Since critical speed can also be affected by shaft straightness and assembly alignment, it is recommended that the maximum speed be limited to 80% of the calculated valve.