Sizing is the most basic and essential factor in hydraulic hose selection. Sizing specifications for hydraulic hose include inner diameter, outer diameter, and length.
- Outer diameter (O.D.) measures the entire span of the hose cross section, including the tube, reinforcement, and cover. The difference between the O.D. and I.D. is the thickness of the hose wall. Outer diameter is important for sizing industrial hose to appropriate fittings and system components.
- Inner diameter (I.D.) is the span of the empty space in a circular hose cross section that affects the flow velocity of the fluid within the hose. A hose with an oversized I.D. will result in sluggish system performance, while an undersized I.D. can cause excessive pressure drops, leaks, and system damage.
- The length of hydraulic hose defines its maximum reach between end-points. Increasing length can also increase pressure drop in the system because hose length is a function of head loss due to friction.
What is dash size?
People new to the hydraulic industry often ask what hose dash size means. Simply put, a dash size refers to a I.D. of a hydraulic hose or fitting in 1/16" increments. For example, -6 hose would indicate a hose with I.D. of 6/16" - or 3/8".
Hoses are measured by inside diameter, unlike tubes which are measured by outside diameter. As a matter of fact, that is how the hoses and tubes are differentiated.
How to determined dash size for your specific hydraulic application
By using a standard nomogram, we can find the relationship between three variable quantities using simple geometric construction. In this instance, our variables are Dash Size (Hose I.D.), Desired Flow Rate and Recommended Flow Velocity. If any two of these factors are known, you can easily determine the third factor.
How to use the nomograph shown below:
- Pick the two known values.
- Lay a straightedge to intersect the two values.
- Intersection on the third vertical line gives the value of that factor.
For example, if we know the Desired Flow Rate is 16 gallons per minute and our Recommended Flow Velocity is 20 feet per second, we can mark these numbers in the corresponding left- and right-hand columns. When we lay a straightedge between the columns and draw a line to connect the values, we see where the line crosses the center column graph; where it intersects determines the Hose I.D. / Dash Size. In this case, we would require a 0.625" (5/8") inch (or larger) hose. The nomogram is an easy and effective tool for helping size your hydraulic hoses.
Article contributed by Kyri McDonough, marketing services manager at Hose Products Division, Parker Hannifin.
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