{"id":4080,"date":"2026-10-01T09:00:00","date_gmt":"2026-10-01T01:00:00","guid":{"rendered":"https:\/\/prancehydraulic.com\/?p=4080"},"modified":"2026-10-06T15:20:38","modified_gmt":"2026-10-06T07:20:38","slug":"hydraulic-motor-case-drain-line-sizing","status":"publish","type":"post","link":"https:\/\/prancehydraulic.com\/es\/blog\/hydraulic-motor-case-drain-line-sizing\/","title":{"rendered":"Normas de dimensionamiento y conexi\u00f3n de la l\u00ednea de drenaje del caso del motor hidr\u00e1ulico"},"content":{"rendered":"
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A hydraulic motor case drain line must carry the motor's leakage and any specified flushing flow to an approved low-pressure destination without exceeding the exact motor's allowable case pressure. Size it for the worst operating condition, including cold oil, peak speed, reversal, elevation and shared-line pressure. The work-port hose size is not a reliable substitute for a drain calculation. A larger hose also cannot correct a blocked fitting or a pressurized tank connection.<\/p>\n

This guide explains how buyers and engineers can specify a drain route, compare candidate line sizes and validate the installation. It does not assign a universal hose diameter or case-pressure limit. Those values depend on the motor configuration and machine circuit.<\/p>\n

\"Fixed
Fixed Displacement Motor A2FM: a catalog product view for identifying the housing, shaft and connections. Confirm the ordered configuration using its drawing.<\/figcaption><\/figure>\n

Separate the case drain from the working return<\/h2>\n

The working return carries the main flow leaving the motor. The case drain normally removes leakage entering the housing and may also carry intentional cooling flow. Connecting the two paths can expose the housing to return-filter restriction, valve pressure drops or pressure generated by another actuator. Whether a shared connection is permitted must be established from the selected motor's instructions.<\/p>\n

Some orbital motors have arrangements that route internal leakage toward a working port, while other configurations require an external drain. A visible third port does not prove that every variant uses the same plumbing. Identify the full model code, drain-port designation and installation orientation before selecting fittings. A plugged port in a sales photograph must never be interpreted as permission to operate with a blocked drain.<\/p>\n

Establish the design flow before choosing a hose<\/h2>\n

Obtain the manufacturer's leakage or drain-flow information at the actual pressure, speed and oil temperature. Ask whether published flow includes flushing flow or only internal leakage. In a circuit with a flushing valve, cooling flow can exceed normal leakage and dominate the drain calculation. Record both continuous flow and the highest credible short-duration flow.<\/p>\n

A worn motor may produce more leakage than a new motor, but an excessively large allowance can hide a component already requiring repair. Use a justified design allowance and a maintenance trigger separately. For an existing machine, measure flow under controlled duty and compare it with model-specific acceptance data. Measurements from another motor size or from unloaded operation are not sufficient.<\/p>\n

Drain flow changes with operating condition. Warmer, thinner oil may increase leakage, while colder oil can create greater line resistance even if leakage is lower. Consequently, the condition producing maximum flow and the condition producing maximum pressure loss may differ. Check both rather than selecting one supposedly worst-case operating point.<\/p>\n

Build a pressure budget for the complete route<\/h2>\n

The available line-loss budget is the permitted case pressure minus pressure already present at the destination and any other relevant pressure contribution. Tank gas pressure, elevated reservoir level and shared return pressure can consume part of that budget. Use one pressure reference throughout: gauge and absolute pressure values cannot be mixed in the same subtraction.<\/p>\n

Include straight hose, rigid tubing, adapters, elbows, quick couplers, isolation devices and the tank entry. The smallest actual bore can control the outcome. Two hoses with the same nominal size may have different inside diameters, and compact elbows can create substantially different losses. Obtain component pressure-drop curves rather than treating every fitting as an unrestricted extension of the hose.<\/p>\n

Where the manufacturer requires an independent drain, retain it as an independent connection. Do not add a filter or check valve simply because it seems helpful. Either device can raise case pressure, especially during cold starts. A drain filter or valve is acceptable only when the complete arrangement, opening behavior and pressure loss have been approved for that motor.<\/p>\n

Use calculations as screening tools, then verify<\/h2>\n

For a preliminary straight-line check, velocity is flow divided by inside cross-sectional area. In consistent SI units, v = Q\/A and A = pi times D squared divided by four. The pressure-loss relationship also depends on viscosity and flow regime; a low-velocity line is not automatically a low-loss line when cold oil is involved.<\/p>\n

For fully developed laminar flow in a straight round tube, the Hagen-Poiseuille relationship illustrates the strong diameter effect: pressure loss is proportional to viscosity, length and flow, and inversely proportional to diameter to the fourth power. This relationship is not a substitute for hose, fitting or transient data. Flexible hoses, entrances and directional changes need their own allowances or supplier curves.<\/p>\n

For an illustrative velocity calculation only, 4 L\/min is approximately 0.0000667 cubic meters per second. In a 10 mm inside-diameter passage, area is approximately 0.0000785 square meters, giving about 0.85 m\/s. That calculation does not establish that 10 mm is suitable. Suitability still depends on route length, viscosity, fittings, tank pressure and the motor's actual limit.<\/p>\n

Compare route choices with a decision table<\/h2>\n\n\n\n\n\n\n\n\n\n\n
Route condition<\/th>\nWhat to calculate or check<\/th>\nPractical response<\/th>\n<\/tr>\n<\/thead>\n
Long hose run<\/td>\nActual inside diameter, length and cold-oil resistance<\/td>\nCompare larger-bore routes using supplier loss data<\/td>\n<\/tr>\n
Multiple compact adapters<\/td>\nMinimum passage size and individual losses<\/td>\nRemove unnecessary restrictions where the approved layout allows<\/td>\n<\/tr>\n
Shared tank manifold<\/td>\nPressure produced by other circuits<\/td>\nTest simultaneous duty or provide the required independent return<\/td>\n<\/tr>\n
Flushing flow present<\/td>\nLeakage plus intentional cooling flow<\/td>\nSize for the combined flow, including switching conditions<\/td>\n<\/tr>\n
Pressurized reservoir<\/td>\nDestination pressure and measurement reference<\/td>\nDeduct that pressure from the available case-pressure budget<\/td>\n<\/tr>\n
Quick disconnect required<\/td>\nCoupler bore, mating compatibility and failure behavior<\/td>\nVerify connection before startup and prevent operation when disconnected<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

The table is a route-review checklist, not a list of universal pass\/fail limits. If a value is unknown, obtain it rather than assigning a convenient assumption.<\/p>\n

Confirm orientation and retain the required oil fill<\/h2>\n

Drain-port location can affect whether the housing remains filled. A manufacturer may specify the highest available port or a particular arrangement for the installed orientation. Follow the exact instructions for horizontal, vertical or inverted installation. A port that is uppermost on a bench may not remain uppermost once the machine is assembled.<\/p>\n

Check how the motor is filled before startup and how oil can drain away during storage or shutdown. Avoid designing an accidental siphon or empty housing. Conversely, do not create an unapproved restriction to retain oil. The correct solution is the specified port and routing arrangement, with the required prefill and commissioning procedure.<\/p>\n

Measure case pressure without changing the result<\/h2>\n

Install a suitable test point close to the case connection and use an instrument able to capture relevant pressure excursions. A gauge located only at the reservoir does not measure pressure at the housing. A slow indicator may miss the brief peak occurring during a rapid reversal or when another actuator returns oil into a shared manifold.<\/p>\n

The measurement arrangement must not introduce a restrictive temporary hose, undersized flow meter or partially closed valve. Document the test equipment's pressure-drop contribution. For flow measurement, keep the housing connected to its approved destination and follow the instrument manufacturer's installation instructions. Discharging oil into an open container can change the circuit and is not a valid substitute for every installed test.<\/p>\n

Commission through cold start, reversal and sustained duty<\/h2>\n

Start with the approved oil fill, correct connections and supported load. Increase speed and pressure in controlled stages. Record oil temperature, motor speed, work-port pressures, drain flow and case pressure together so a peak can be tied to an operating event. Include both rotation directions where the machine uses them.<\/p>\n

Repeat the checks during realistic cold startup and after reaching operating temperature. Test simultaneous machine functions if they share plumbing. A pass with one motor running alone does not validate a manifold that becomes pressurized when another function is activated. Stop if leakage, unexpected temperature rise or case pressure approaches the approved limit.<\/p>\n

Put the drain requirement into the RFQ<\/h2>\n

Send the supplier the motor code, hydraulic schematic, mounting orientation and a sketch of the complete drain route. Include hose inside diameter and length, fitting part numbers, reservoir connection height, oil grade and temperature range. Specify whether a brake or flushing valve is fitted and ask which flows must be included.<\/p>\n

Request a written case-pressure limit with its continuous and transient qualifications, the approved drain-port location and the startup oil-fill procedure. Ask whether the proposed destination and any filter, coupler or valve are permitted. Keep the response with the machine commissioning record rather than relying on a verbal reassurance that the hose looks large enough.<\/p>\n

\"Fixed
Fixed Displacement Motor A2FM: a catalog product view for identifying the housing, shaft and connections. Confirm the ordered configuration using its drawing.<\/figcaption><\/figure>\n

Frequently asked questions<\/h2>\n

Can I use the same size hose as the motor return line?<\/h3>\n

Not as a sizing rule. The two lines carry different flows and have different pressure limits. Calculate the drain route from its own flow, viscosity, length, restrictions and destination pressure, then compare measured case pressure with the selected motor's instructions.<\/p>\n

Does a larger drain hose always prevent shaft-seal leaks?<\/h3>\n

No. A blocked coupler, incorrect port, pressurized tank, damaged seal or shaft runout can remain the real cause. A larger hose addresses line resistance only. Inspect the full path and measure pressure before assuming that hose diameter caused the leak.<\/p>\n

Should a case-drain filter be installed?<\/h3>\n

Only if the motor and circuit instructions permit the arrangement and the loss budget covers its cold and loaded condition. Adding filtration without checking restriction can create the case-pressure problem it was intended to prevent.<\/p>\n

How do I know whether flushing flow is included?<\/h3>\n

Check the motor and valve documentation and ask the supplier explicitly. Record whether the published drain value describes leakage alone, leakage plus flushing, or a test condition different from the planned machine duty.<\/p>\n

Can a drain be shared by several motors?<\/h3>\n

Possibly, but only with an approved arrangement and a verified pressure budget at each housing. Evaluate simultaneous flows and transients. If the manufacturer requires individual drains, a common manifold must not override that requirement.<\/p>\n

Product and related engineering resources<\/h2>\n

For a component enquiry, review Fixed Displacement Motor A2FM<\/a> and provide the full specification and installation conditions. Related guides: piston motor range<\/a>, orbital motor range<\/a>, motor case-pressure checks<\/a>, motor drain-flow measurement<\/a>, motor efficiency calculation<\/a>, Hydraulic Motor Shaft Seal Failure: Pressure, Alignment, and Wear<\/a>, Hydraulic Motor Factory Test Report: Torque, Speed, and Leakage<\/a>. Product photographs illustrate catalog configurations; they do not establish a pressure, speed or shaft-load rating for an unconfirmed order.<\/p>\n

University lecture: hydraulic motor fundamentals<\/h2>\n

NPTEL-NOC IITM presents a hydraulic-motor lesson from IIT Madras. The lecture supports the distinction between hydraulic input and shaft output used in this guide. Component-specific installation and operating limits still require the selected motor documentation.<\/p>\n