Two suppliers can offer the same displacement and put very different evidence behind their quotes. Before you approve a low speed high torque hydraulic motor, ask each supplier to show that the offered build can start the load, hold the required crawl speed and stay inside its limits through a normal shift. Put the three jobs on separate lines. They need different evidence, and the motor may do two of them well while the third still needs an answer.
Start the comparison with the complete order code and the drawing. Then put the evidence next to each machine condition. A family curve can help narrow the size. For a result marked measured, find out which sample was on the bench and whether the shaft, ports and brake match the offer. Keep that sample code on the receiving record too.
Start with the event that made the buyer call
Imagine an OEM needs a motor for an indexing table. The table restarts with a load already on it, creeps while it locates the next position and then spends most of the cycle at a higher speed. The purchase request says “low speed, high torque, 250 cc.” The supplier replies with a good running point at 80 rpm. That reply settles one question if the conditions match. It says little about the instant the table first moves or the interval where the speed must stay close to 4 rpm.
I would put the machine events on the request in the order the operator sees them. First comes a restart after a normal stop, with the load still on the table. Then comes the short positioning move. The last line is the longer running interval. There may also be a separate restart after the machine has sat long enough for the oil to cool. If the control can move in both directions, record which direction has the harder start or ask the reviewer to cover both. This is a paper example, so the speeds identify the work to discuss. They are no reason to run a real sample outside its documented range.
Machine event
Record from the machine owner
Evidence to ask for
Restart with the load in place
Shaft breakaway torque or an approved load estimate, pressures at the motor and direction
Loaded starting result for the offered code and applicable conditions
Slow positioning
Target shaft rpm, allowable variation and time in that part of the cycle
Loaded speed record over a defined observation window
Longer running interval
Shaft rpm, load torque, operating temperature and how long it runs
Applicable performance curve or data at the running point
After an overnight stop
Starting oil temperature, brake sequence where applicable and load at first motion
Approved cold condition or an agreed test limit
A purchase team can work with a few empty boxes. Write “not measured” if nobody has measured the loaded start, then put the person responsible for supplying the value beside it. Leaving a cell blank and carrying over the biggest catalog torque gives the next person no clue that the most important load is still open. A photograph of the table or old motor will help identify the hardware. The person who needs to compare the quotes also needs the load at the motor shaft or enough information to calculate it.
A start can be unrepresentative even if the sample runs on the actual machine. If an unloaded table is all the workshop can use, put that in the report header. It proves something about the installation and controls at light load. When the same table starts with a fixture or product on it, bearing friction, drive load and any brake release sequence can be different. Write which result belongs to which condition. A short video is helpful when it shows the event and the trace or report lets the reviewer connect it to that exact sample.
Ask which kind of torque the supplier quoted
Read the heading above the number. A supplier might provide theoretical torque, running torque at one point, or a minimum starting torque measured or specified under a named pressure condition. They answer different questions. At rest the speed is zero and a running efficiency picked from a catalog curve does not establish what the motor can break away against. An unloaded starting pressure is also a different measurement. It describes the pressure needed to begin turning a motor with no specified external shaft load.
Take a hypothetical quote for a 250 cm³/rev motor. At first motion one work port is at 145 bar and the other at 15 bar. The pressure drop available across the motor is 130 bar. With no mechanical loss, the torque calculation is 130 × 250 ÷ (20π), or about 517 N·m. Suppose the machine needs 430 N·m at breakaway. The difference between 517 and 430 is a screening observation. Part of that difference is needed for losses, and the calculation does not show how much will be available at the worst starting condition. That is the line on which to ask for loaded evidence.
If the quotation used 145 bar as its pressure drop, its theoretical result would be about 577 N·m. Both figures could appear in a spreadsheet without a broken formula. The input to one formula is wrong for the stated ports. Save the two pressure readings as well as their difference. If a pressure tap is far away from the motor, put its location in the file. A pump outlet gauge may include the pressure spent across a valve or along a line before the oil gets to the drive. The return port pressure belongs in the subtraction even when the tank is open to the atmosphere.
On the paper review for this example, the supplier could take the 430 N·m requirement as the defined load and respond with the condition at which its offered motor can move it. Or the supplier could propose a test method that delivers a comparable startability result. In either case, the report needs to say how the movement was detected and how the pressure and load were established. ISO lists two methods for determining the startability of rotary hydraulic motors in its standard on this subject. If the parties use a named method, agree on it in writing. A screenshot of a test bench does not establish the method it used. ISO 4392-2 startability
It is also worth asking which start was captured. A sample that moves readily from one stopping position may behave differently when it stops elsewhere in its cycle. In a rotary hydraulic motor, the torque available at first movement can depend on where the internal elements and shaft stopped. The purchase team can ask for the procedure and how it handles starting position. Where the machine runs in both directions, name the direction for each result. Keep the worst observed result visible rather than filing only the cleanest trace.
A start number and a running number can both be valid on the same supplier sheet. Write the test conditions next to each and read them in sequence. If the quote says it can make a high torque briefly, ask whether the machine starts once at the beginning of a shift or restarts repeatedly. A working envelope with a time rule is read against the actual duty. A statement that the motor can reach a load once does not establish what it can repeat all day.
Put the crawl test beside the drawing
If the table must hold 4 rpm for a positioning operation, a display that says “4” once during deceleration is little help. Agree on the load and the length of the observation first. Describe what the drive needs to do while it is turning slowly. For one machine, a speed swing during the move may put the fixture outside its permitted stopping window. For another, only the average through a longer feed matters. The acceptance sheet should say which of those jobs the machine has.
The measurement needs a useful resolution. At 4 rpm, the shaft makes one revolution in 15 seconds. A pickup that provides one count per revolution will give too little information about a brief change inside that turn. A higher resolution trace, provided and used under the approved test method, may show a stall or lurch which a single rev count hides. If the shop already has a controller trend, record what is being averaged. Ask for the original data if the summary value cannot answer the positioning question.
A test setup can introduce its own speed variation. Put the valve or other method of regulating flow on the bench sketch. Keep its command alongside shaft speed and load. If speed hunts while the valve command is changing, give the engineer the two traces. If the bench cannot hold a useful low flow steadily, say which point was beyond its ability. An unsteady trace is worth investigating, but deciding that the motor caused it requires the bench and load information too.
Danfoss gives a practical example of reading a curve with its conditions attached. The OMT and OMV technical information describes performance diagrams from representative units, with oil viscosity, temperature and return pressure stated. A buyer can use that format to frame a request for another supplier: which units does this curve represent, what was the return condition, and where on the chart is my point? The published Danfoss diagrams apply to the models and conditions in that document. The offered sample needs evidence for its own build. Danfoss OMT and OMV technical information
To make the review usable, record a small series of readings rather than leaving “smooth” on the sheet. The table below is an example of what to ask the test team to supply. Set the tolerances before the first run. If the acceptance requirement is based on motion at the machine output, give the reduction between that output and the motor. A number measured on the wrong side of a gearbox can make a good sample look slow or a bad one look acceptable.
Trace field
Where it belongs on the bench record
Motor speed
Sensor location, units, resolution and any controller averaging
External load
Load device and torque measurement point, with gear ratio if used
Hydraulic pressure
Two work-port readings and any separate drain reading relevant to the build
Fluid
Grade, temperature at the recorded point and viscosity where established
Test interval
Start of the crawl window and how the minimum and maximum speed are read
Direction
Which way the shaft is viewed and which port is driven
Stop behavior
A separate note for any stall or stop during the requested interval
The point of the sheet is to preserve enough of the bench setup to explain the next result. Suppose the first plot has two speed dips. Mark their times. Check whether both occurred just as the load changed or one was captured while the drive was still slowing into the window. A slow interval with the load held constant is different from a transition into it. Keep both if the machine needs to work through both. The engineer should be able to tell which segment the buyer used when the order was approved.
Make two supplier reports comparable
I put the report number on the same line as the sample code. One file might have a chart and a complete code. The other could have a tidy list of readings with nowhere to write the serial. Go back to the bench sheet in the second case. There may have been two units on the rack that day. Ask which one ran, which shaft it carried and whether the report follows that exact assembly. Keep a family curve in the file too. Put it under the model family where it can answer a sizing question.
The two fictional replies make this easy to see. A describes a loaded run at a 130 bar difference, with 50°C oil, the direction specified and a serial number at the top. B sends “start at 160 bar” in an email. I'd keep 160 in the file and reply with three blanks. What was on the shaft? What was the oil condition? Can B show the same loaded start at the pressure the machine actually provides? If the two benches cannot run the same conditions, have the engineer decide which figures can still be compared.
The worksheet can carry N·m in the comparison column and lbf·in in a column off to the side. Keep the number as supplied there. When the result sits right on the limit, I would ask how the test team knows that its instrument can distinguish the two sides of the line. The calibration and the spread across repeat runs are more informative than a conversion carried to four places.
Circle 80 rpm on the supplier's running curve and follow it to the torque the table needs. You want the point where the two meet, with the temperature and pressure conditions the curve gives. A large number on the far end of the speed axis can belong to another load. The point you circled needs the proper time band as well. If it is only allowed intermittently, compare the permitted duration with the production schedule and ask what the oil temperature was when it was checked.
I would put the report beside a simple comparison card with three entries for each supplier. Under the first, write the part of the machine duty the supplier has actually covered. Under the second, write the exact build and drawing revision. The third holds the remaining questions. A statement such as “minimum torque at requested condition not supplied” is much easier to close than a red mark in a general quality column. The next email can ask for one missing line instead of inviting another brochure.
Review the shaft and mounting before the loaded test
On the layout drawing there's a distance I'd ask for before pricing the motor. It runs from the mounting face to the centre of the sprocket. Next to it, draw the direction the chain pulls and show any support bearing. The supplier can then check what the shaft sees besides the torque. In the test room that same unit may be joined to a load cell by a straight coupling. The two sketches belong in the file because the installed chain load will not be present in that bench arrangement.
Lay the mating coupling on the bench next to the motor drawing. Where does the shoulder sit when it is assembled? A shaft can have the expected number of splines and still end short of the engagement the drawing calls for. Read the pilot and the shaft length from the mounting face specified on the drawing. Add a separate mark for any retaining thread and its hardware. That marked view gives the shop one reference to measure from when the crate is opened.
The spare fitting on the workbench is a poor thread gauge. It can start in a port and still have the wrong seat. Write the full port callout on the receiving sheet and keep the specified fitting beside it. If the proposal moves the connection to the end cover, take the drawing out to the bracket. There might be room for the body and no room left to turn the hose. Put the bend in the drawing before signing off the fit.
On the circuit print, I would trace the drain back from the motor and label it in a different ink from the two work lines. Put its pressure requirement beside the seal option on the drawing. If the motor arrives with a brake, find the release line as well. The brake's holding requirement and its release condition go on their own lines. The machine maker can then check the approved sequence without mistaking a pressure on the main circuit for a reading at the brake or in the housing.
Start the family check in the BLINCE orbital hydraulic motor range. For a machine that needs another architecture, the BLINCE hydraulic motor range shows the other families. I'd copy the candidate's full code onto the order only once the drawing has named the shaft, ports and any drain or brake arrangement. This is where a particular mounting or return condition can send the buyer back to the family list.
Decide whether a sample is ready for the next order
On a receiving sheet I would give identity and duty their own boxes. A crate might contain the right motor, with all the drawing numbers in order, while the workshop still lacks a loaded start result. Another sample could run steadily on the bench and turn up with a side port where the production drawing calls for an end port. Sign the box you can sign. Write a request for the missing test or drawing beside the other one.
Six months later the second crate reaches the warehouse. The motor looks like the approved one. I'd want the receiver to find the long code on the label and compare it with the code and drawing saved against the stock number. If a new plate moves the hose connection, send that detail to the buyer before filing it under the old approval. The assembly can still have the original displacement.
A bench and a production machine can have two different jobs. In our table example the bench can show what the motor does against a named load at the slow point. The machine can show how that result behaves behind the actual valve during a positioning move. If the bench graph is steady and the installed drive lurches, put the two circuit sketches next to the plots. The line where the load, valve or gearing changes gives the engineer somewhere to begin.
On the line below the sample result, put what the next shipment is expected to include. The order may ask for a drawing and a label check on each crate, or for a defined test record where the start duty calls for it. Keep the agreed sampling or inspection plan with the buyer's copy. The first report describes a particular motor. A repeat order needs its own agreement about how the relevant checks will follow later units.
Keep the observed range on the receiving sheet
There is one extra field I'd add to a loaded report for a reversible machine: the direction viewed from the same end on both drawings. A stamp reading CW can describe the motor shaft on one sheet and the machine output on another. If a gearbox reverses the movement, the two arrows will point differently. Draw both. Then put the starting load beside the arrow it belongs to. A return stroke with no load should not be used to fill the loaded direction's result. File the easy start too, because it can help the test team see why the two runs differ.
On a made-up report the three starts came at 126, 129 and 133 bar. The load and return condition were held alike. Our machine can supply a 130 bar drop for that start. Circle 133 and ask how it was obtained. A mean would leave that result out of sight. Get the instrument details and the position and direction of each trial. If a particular stop position needed more pressure, the application engineer has a useful question to take back to the machine.
Our hypothetical locating move lasts nine seconds. The supplier sent an average for one minute. There is room in that minute for a low dip right where the locating pin needs steady travel. Ask which nine seconds belong to the move and what the original trace says. The long average may still tell you something about the feed. File it as the mean, and keep the short window open if it has not been measured.
What to put in the first supplier email
Give the supplier the machine task in one sentence. Then send the required torque and speed at the motor for start, crawl and normal running, with the available pressure drop and return conditions. Attach the drive and mounting drawing or photos with reference dimensions. Include the operating temperature range, fluid, any brake or drain arrangement and the sample quantity. If one requirement comes from a calculation, name the assumptions. This leaves the supplier enough to respond with a code, drawing and a proposed verification plan.
I'd finish the email with one question: which of our three machine conditions can you cover with this exact build? Save the answer beside the drawing and the report number. On a later order, those three things should tell the new buyer which operating points were cleared. There is no reason for the buyer to reconstruct the original call from a note saying “sample OK.”
I'd mark the missing direction in the quote file and write the load required beside it. That makes it possible to reopen one item later without rereading the whole selection. Once a result arrives, file it with the order code and the pressure at the test. The bench evidence, the receiving record and the machine trial can then tell the same story.
This article is a general engineering guide. Final component selection should be based on machine drawings, measured hydraulic data, working conditions, safety requirements, and confirmation from a qualified hydraulic engineer or supplier.
Blince Hydraulic Team
Blince Hydraulic is an industry-leading company dedicated to precision-engineered fluid power manufacturing and custom hydraulic solutions. Backed by decades of deep field expertise in industrial machinery and thousands of successful global deployments, our engineering team focuses entirely on high-performance hydraulic component manufacturing, including specialized orbital motors, high-pressure travel drives motor, and robust directional control valves. Our production infrastructure utilizes state-of-the-art multi-axis CNC machining systems and is fully ISO 9001 certified to guarantee repeatable volumetric accuracy across every single manufacturing run.
We deliver fast, highly dependable, and cost-efficient hydraulic solutions to heavy industry distributors, machinery OEMs, and maintenance crews across more than 150 countries. Whether your active project calls for a small-volume batch of customized shaft profiles or a large-scale production run of severe-duty cast iron gear pump, we configure our flexible production schedules to meet your target lead times with total pricing predictability. Partnering with Blince means securing maximum system efficiency, elite material quality, and uncompromised fluid power professionalism.
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