Importance of Properly Sizing a Rotary Index Table

Sizing a rotary index table properly is critical for several reasons, particularly in precision machining, automation, and manufacturing applications. The rotary index table is used to position workpieces at precise angles for operations like drilling, milling, or inspection. Correct sizing ensures optimal performance, safety, and cost-effectiveness.

When sizing a rotary index table, many customers begin with a 3D model of their tooling created in design software. While it’s possible for the software to provide the inertia of the mass properties, it’s ultimately up to the designer to ensure the output is accurate. In most cases, designers may lack the experience to properly validate these values, which can lead to discrepancies. It’s crucial that the software is set to select only the rotating components of the index drive and that the components have the correct material properties and densities applied. For instance, using the density of steel instead of aluminum for a dial plate can cause the calculated inertia to be double what it should be. Additionally, placing the World Coordinate System (WCS) anywhere other than the center of rotation in the 3D environment can result in highly inaccurate properties, as this would skew the mass and center of gravity values, which may appear far off from the intended location.

At Motion Index Drives, we use the mass properties provided by the customer’s design software as a starting point for sizing the application. However, we don’t rely solely on these values. As a trusted partner, we meticulously review each application and cross-check the software’s inertia calculations with the mathematical formulas for inertia to ensure that the loading is accurate. This extra step ensures that the index drive is neither undersized, which could lead to premature failure, nor oversized, which would result in unnecessary costs for both the index drive and the control system.

Four Items to Consider When Sizing a Rotary Index Table

1.   Swing Diameter

The swing diameter should generally maintain a relationship with the cam follower pitch diameter of 8:1 or less, and never exceed 10:1.
For example:

• For a 2000mm application, divide by 8:
  • 2000 / 8 = 250 → Size the table as RT250.
• For 1800mm:
  • 1800 / 8 = 225 → Size the table as RT250.
• For 1600mm:
  • 1600 / 8 = 200 → Size the table as RT200.

The maximum recommended swing diameter for each model can be found in the catalog for each product.

2.   Load Inertia

Determining load inertia can be complex. To estimate worst-case inertia, multiply the total load by the square of the radius. For example:

• If the radius is 1.5m and the load is 1,500kg, the inertia would be:
  1,500kg∗(1.5m)2=3,375kg⋅m2

Using this estimate, you can calculate the required index time for a specific model. For example, an RT400-4 stop with this inertia would require an index time of 3.82s. If possible, use the center of gravity radius for more accurate calculations. Additionally, Motion’s internal TCC calculator can help estimate inertia.

3.   Indexing Time

The longer the indexing time available, the greater the inertia the index table can handle. If the application allows for a slower indexing time, this can help reduce the required size of the indexer and make the system more cost-effective. Slower indexing also reduces the stress on the table.

4.   Load Weight

If index time is not a critical factor, the total load weight must be checked to ensure it doesn’t exceed the bearing capacity. Bearing capabilities for each model are listed in the catalog.

Important Considerations for Sizing Trunnions and Rotary Index Tables

When sizing trunnions, it is important to consider the factors mentioned earlier, but an additional consideration is the out-of-balance (static) torque. Static torque often becomes the deciding factor when sizing a trunnion. This type of torque arises from any imbalance in the system and must be accounted for to ensure proper performance.

Key Points for Sizing Flexible vs. Fixed Index Drives

Flexible Index Drives

These drives are stronger than their fixed counterparts because they always have a minimum of two cam followers engaged in preload. In the case of TMF indexers, up to five cam followers can be in contact with the cam at any given time, which contributes to their increased strength and reliability.

Fixed Index Drives

For preliminary estimates, assume the same size for flexible and fixed rotary index tables until the final loading can be calculated by engineering. This initial sizing provides a starting point for further refinement.

Sizing Parallel and Right-Angle Rotary Indexers

For parallel and right-angle (globoidal) rotary indexers, output torque is typically the most critical factor when sizing the unit. To size these types of indexers, it’s important to gather the following key information from the customer:

• Required output torque
• Cycle rate (how fast the system needs to operate)

By following these guidelines, you can ensure that the rotary index table or trunnion is properly sized for its intended application, optimizing both performance and efficiency.

How Motion Index Drives Engineered a Rotary Index Table with Frame

In the example below, the customer’s fixtures and the center weldment frame have a combined weight of approximately 3000 kg, representing the total moving mass. The customer estimates that the distance from the center of rotation to the center of the rotating tooling mass is 0.927 meters. To calculate the mass moment of inertia for the tooling fixtures and rotating structural frame, we used the following formula:

I=(0.927)²×3000kg=2577kg⋅m²

Motion Index Drives incorporates a safety factor into the calculated mass moment of inertia to account for any potential miscalculations in weight distribution or inaccuracies in the application of mass properties during the design process. In this instance, we used a conservative value of 3000 kg·m² for determining the appropriate RT series fixed indexer and for properly sizing the motor power.

Choose Motion Index Drives and Be Confident That You’re Getting the Perfect Size for Your Application

The correct sizing of a rotary index table is crucial for maintaining operational precision, enhancing machine efficiency, ensuring safety, and maximizing cost-effectiveness. At Motion Index Drives, we specialize in engineering custom rotary indexing solutions tailored to your unique manufacturing needs. With our expertise, we can help you optimize your machining processes, minimize downtime, and extend the lifespan of both your rotary table and the equipment it’s integrated with.

Let us design a solution that fits your specifications perfectly, so you can achieve the highest level of performance and productivity. Contact us today to discuss your project and learn how we can help you get the most out of your rotary indexing systems.