Routing rules enhance system reliability

July 22, 1989
Following some basic guidelines for routing and connecting tubing and hose in pneumatic and hydraulic applications offers several design benefits.

Following some basic guidelines for routing and connecting tubing and hose in pneumatic and hydraulic applications offers several design benefits. These include faster installation, easier servicing, less chance of leakage and, potentially, longer service life and lower costs.

Most components do not have an exact life expectancy. But improper assembly and installation almost always degrades service life or system performance. Following these simple routing rules helps ensure optimum operations.

Fewer joints = fewer leaks. Simply put, fewer fittings on an application means fewer chances of leaks. The goal is to plan the routing to function efficiently without unnecessary parts. For example, instead of a straight hose fitting and a 90? adapter, use a 90? hose fitting. This eliminates a potential leak path and reduces overall costs.

Sometimes, a nonstandard fitting may be required. In most cases, the parts are readily available and may not cost much more than standard items. Even if initial costs are higher, they are likely still less than the cost of the two fittings being replaced, with the added benefit of improved service life. A common mistake is to overdesign a system with more components than necessary.

Optimize the line size. When in doubt, most people believe bigger is better. This type of thinking often drives up installation and operating costs. Perform the necessary calculations or use nomograms to properly size lines. Usually a designer needs to know the required fluid velocity to determine the recommended maximum values for suction, return, or pressure lines. Such data are normally noted on a nomogram but, if not, use a maximum suction value of 4 fps, 10 fps for return lines, and 20 fps for pressure lines. Proper line size will optimize the efficiency of the entire system and help control fluid temperature. Overheating prematurely degrades the fluid quality and is potentially damaging to seals.

Straight and parallel. Regardless if using pipe, tubing, or hose, the ideal layout should be straight and parallel. Straight lines reduce the number of fittings and do not create obstacles that hinder flow. The added bonus is aesthetic. The rule of thumb is that if it looks good, then it probably is good.

Keep it straight for two diameters. Hose is measured by ID, pipe and tubing by the OD. A good rule of thumb is that the line exiting the fitting should run straight for a minimum of two IDs for hose and two ODs for hard pipe. The hose fittings' shell and nipple have grooves or barbs that grip the hose. Bending too close to the fitting creates excessive stress on the hose structure that can result in premature failure. Bending too close to pipe and tubing yields a similar result.

Hose versus tubing. There are numerous factors to take into consideration for this decision, which should be made on a case-by-case basis. A few considerations to keep in mind include:

• Is there room for hard piping?
• How do costs of hose and tube compare?
• Does the hose meet pressure requirements?
• What assembly equipment is required?
• What is the operating environment?

While many other factors can come into play, the important consideration is that planning ahead will save a lot of aggravation later on.

Know the duty cycle. Duty cycle is an important factor for plumbing any system. This helps determine the type of hose or tubing, fittings, and the need for additional clamping.

Design for assembly and maintenance. Keep in mind the assembly and maintenance personnel during the design stage. Being able to easily install parts saves time and money. It is also critical to design system components into readily accessible areas, so they are fairly simple to replace in the event of a failure. A relatively minor change in location or attachment method may pay substantial dividends in lower maintenance costs.

Make it repeatable. Designing components into a convenient location can also help make a system repeatable. A pneumatic or hydraulic system with a simple and straightforward design is more readily built repeatedly the same way in production and more readily serviced by a range of personnel.

Information for this story provided by Ron Mramor, Technical Services Engineer, Parker Fluid Connectors Group, Cleveland, Ohio.

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