Machine Design

A better way to size bolts

Just about every product has several parts connected by bolts, screws, or pins.

CosmosWorks requires that users identify a few items such as faces the bolts will contact, a bolt diameter, and material. The bolt-sizing features appear in CosmosWorks 2005 and soon in CosmosDesignStar 2006.

Software calculates information such as axial force, shear force, and bending moments to size the bolt.

A close up of a hole in the valve flange shows where the software places beam and bar elements. The beam (boltshank) element is connected by bar elements to all nodes on the bolt and nut-contact face.

One assembly task often calls for sizing bolts and finding the effect of the connection on the whole assembly. Analysis experts generally acknowledge that it's difficult to model bolt behavior in a finite-element simulation, as indicated in several recent articles (MACHINE DESIGN, December 12, 2004 and February 3, 2005).

"Simulating bolts in finite-element assemblies requires a great deal of knowledge and time," says Ramesh Ramalingam, a product manager with Solid-Work's Cosmos division in Los Angeles.

"Those doing analyses are asked to do more and so need help with these problems. The good news is that the latest simulation software handles a lot of the former complexity. For example, rather than having to manually define bolt behavior, software such as Cosmos, has virtual-bolt connectors that make it easier and faster to analyze assemblies that contain bolts and screws. Virtual connections consider the effects of bolt pretension and shear, especially when the bolt diameter is the same as the bolt-hole diameter in the flange. Users can also find axial and shear reactions at the bolt to determine its size, or whether clamping force generated by the bolt and the tightening torque is sufficient to overcome external loads," says Ramalingam.

The concepts behind virtual-bolt connectors are the same as those used by dedicated analysts. "The program takes no shortcuts in accuracy. Rather, it offers a simple interface, calling for straightforward input, while handling many tasks in software that were performed by analysts," he adds.

Take a large ball valve with flanges, for example. The task will be to find the loads on the bolts. "Users must first identify the bolt shank, nut (when threaded holes are used, nuts won't be), bolt diameter, material, and flange faces that contact the bolt head."

The software computes bolt preload when users supply a tightening torque as well as a torque coefficient to account for thread friction. All these inputs are defined in a single dialog. After reaching a solution, the software provides axial force, shear force, and bending moments acting on the bolt.

A virtual-bolt connector, however, doesn't provide stress distributions on the nut and bolt. Instead, it finds the effect of the bolt on the parts adjacent to it, or on the overall assembly. "When such stress distributions are important, users can model bolt behavior by including the solid models of bolts and nuts, and apply contact conditions rather than the virtual-bolt connector," adds Ramalingam.

The FEA software models the bolt shank with a beam element and connects the contact face between the bolt head and flange with rigid bar elements. "The other end of the bolt shank is similarly connected to the contact face between the nut and flange using another set of rigid bar elements. The software models bolt preload by applying an axial force on the beam. The beam element is a tensiononly element that resists tension but not compression. When the bolt shank and hole area are a tight fit, the software models shear by connecting rigid bar elements between the flange-hole faces and bolt shank," he says.

Ramalingam admits that because rigid bars connect the beam with flange faces, stresses close to the bolt-nut area may not be accurate. "However, this effect decreases gradually until it practically disappears in regions about one bolt diameter away from the bolt faces."

"Bolt connectors simplify bolt modeling, making it easy for novice analysts to work on assemblies with bolts and screws. They don't have to worry about where to put beam elements, what stiffness to assign bars and beams, or what axial force to apply to simulate the tightening torque. In addition, if users model just virtual-bolt connections, they would have to manually define the beam and bar elements," says Ramalingam.

A caution for do-it-yourselfers

If users feel compelled to manually model the behavior of even one bolt in the ball valve, "They would have to define one beam element, 66 bar elements to connect one end of the beam to the bolt contact face, and another 66 bar elements to connect the other end of the beam to the nutcontact face," says Ramalingam. Modeling 16 bolts and nuts in this assembly means users would have to define 16 beam elements and 2,112 bar elements.

"If the bolt diameter is same as the bolt hole in the flange, then users have to define an additional 3,584 bar elements to accurately account for shear. If the design changes, causing a geometry and mesh change, the user would have to repeat these steps each time. And if the user errs in defining a beam or bar element, it will be difficult to isolate the error," he says.

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