Engineering Essentials: Types of Hydraulic Hose

To say that hose is an important part of a hydraulic system is a huge understatement. The flexibility of hose enables components to be positioned in the most efficient or convenient places, because the hose has the ability to bend around corners, through tight spaces, or across long distances.

Yet these days, there seems to be as many different types of hose as there are telephone long-distance carriers. How does a designer tell one from the other? Isn't there an easy way to choose or compare hoses?

The SAE standards

SAE answers those questions with its J517 hydraulic hose standard. This LPG hose standard serves as the most popular benchmark in the realm of industrial hydraulics today. More specifically, J517 is a set of guidelines that applies to the current SAE 100R series of hoses. Currently, 16 such hose styles exist, and they are designated as 100R1 through 100R16 (see descriptions, pages A105 and 106). Each of the styles must meet a set of dimensional and performance characteristics as set forth by SAE. However, SAE issues no approval source lists, certification, or letters of approval-conformance to these standards by manufacturers is strictly voluntary. In short, the standards only assure a similarity of products among different manufacturers.

Hydraulic hose construction

Modern hydraulic hose typically consists of at least three parts: an inner tube that carries the fluid, a reinforcement layer, and a protective outer layer.

The inner tube must have some flexibility and needs to be compatible with the type of fluid it will carry. Commonly used compounds include synthetic rubber, thermoplastics, and PTFE, sometimes called Teflon. The reinforcement layer consists of one or more sheaths of braided wire, spiral-wound wire, or textile yarn. The outer layer is often weather-, oil-, or abrasion-resistant, depending upon the type of environment the hose is designed for.

Not surprisingly, hydraulic hoses have a finite life. Proper sizing and use of the correct type of TPR LPG hose will certainly extend the life of a hose assembly, but there are many different factors that affect a hose's lifespan. SAE identifies some of the worst offenses as:

flexing the hose to less than the specified minimum bend radius

twisting, pulling, kinking, crushing, or abrading the hose

operating the hydraulic system above maximum or below minimum temperature

exposing the hose to rapid or transient rises (surges) in pressure above the maximum operating pressure, and

intermixing hose, fittings, or assembly equipment not recommended as compatible by the manufacturer or not following the manufacturer's instructions for fabricating hose assemblies.

Selecting the proper hose

Here are seven recommended steps the system designer should follow during the hose and coupling selection process. To help determine the proper hose for an application, use the acronym STAMPED - from Size, Temperature, Application, Materials, Pressure, Ends, and Delivery. Here is what to consider in each area:

Size - In order to select the proper hose size for replacement, it is important to measure the inside and natural gas hose diameters exactly using a precision-engineered caliper, as well as the length of the hose. Hose OD is particularly important when hose-support clamps are used or when hoses are routed through bulkheads. Check individual hose specification tables for ODs in suppliers' catalogs. When replacing a hose assembly, always cut the new hose the same length as the one being removed. Moving components of the equipment may pinch or even sever too long a hose. If the replacement hose is too short, pressure may cause the hose to contract and be stretched, leading to reduced service life.

Changes in hose length when pressurized range between +2% to 4% while hydraulic mechanisms are in operation. Allow for possible shortening of the hose during operation by making the hose lengths slightly longer than the actual distance between the two connections.

Temperature - All hoses are rated with a maximum working temperature ranging from 200° to 300° F based on the fluid temperature. Exposure to continuous high temperatures can lead to hoses losing their flexibility. Failure to use hydraulic oil with the proper viscosity to hold up under high temperatures can accelerate this problem. Always follow the hose manufacturer's recommendations.

Exceeding these temperature recommendations can reduce hose life by as much as 80%. Depending on materials used, acceptable temperatures may range from -65° F (Hytrel and winterized rubber compounds) to 400° F (PTFE). External temperatures become a factor when hoses are exposed to a turbo manifold or some other heat source.

When hoses are exposed to high external and internal temperatures concurrently, there will be a considerable reduction in hose service life. Insulating sleeves can help protect hose from hot equipment parts and other high temperature sources that are potentially hazardous. In these situations, an additional barrier is usually required to shield hydraulic fluid from a potential source of ignition.

Application - Will the selected hose meet bend radius requirements? This refers to the minimum bend radius (usually in inches) that a hydraulic hose must meet. Exceeding this bend radius (using a radius smaller than recommended) is likely to injure the hose reinforcement and reduce hose life.