The first thing I'd ask about a speed complaint is where the pickup sits. A display may be reading the drum after the gearbox. The limit in the motor book refers to the motor shaft. With a variable motor there is another question: did it reach the displacement the controller commanded? Write the speed source and the displacement evidence next to the number. They make more difference to the review than the number of decimal places on the display.
I'd call it hydraulic motor overspeed once the speed at the shaft is outside the range the motor maker allows for that particular job. If the printout has three different speed figures, put the circuit and the time rule next to the one you're using. A descent with no change in the pump command is a clue. Follow the load on the drawing. It may have started turning the motor, and the machine still has to control the energy it sends back.
A speed complaint that starts at the wrong end of the gearbox
“420 rpm during deceleration” comes in on a note. The sticker over the pickup says OUT. The motor sheet is for the other end of the gearbox. On this imaginary job the engaged gear takes five turns at the motor for one at the output. The usual 330 on the display goes down as 1,650 at the motor. The 420 entry goes down as 2,100. I'd leave both pairs on the work order and circle the one that came from the descent.
The next blank on the job card is the limit for that event. Before filling it in, check the ratio against the current gearbox drawing. On a two-range transmission an order confirmation may give the ratio for a range the machine wasn't in. If a clutch released or slipped while the speed was high, even the right gear ratio is insufficient for that interval. A direct measurement at the motor would avoid the conversion if the equipment maker provides a safe way to make it.
Speed entry
Example
What the reviewer should verify
Output speed during regular motion
330 rpm
Sensor is on the reduction output
Output speed at the reported event
420 rpm
Record is from the same sensor and range
Ratio while the drive is engaged
5:1
Ratio is defined as motor speed to output speed
Motor speed at regular motion
1,650 rpm
Calculated from the example ratio
Motor speed at the event
2,100 rpm
Compare with the actual motor limit for that event
A winch log can have metres of rope in it rather than revolutions. Which layer was the rope on? At 60 m/min the arithmetic in my notebook would read 60 ÷ (2π × 0.20) for the inner example and 60 ÷ (2π × 0.25) for the outer one. That is about 48 rpm and 38 rpm at the drum, respectively. If somebody uses the bare drum diameter while the rope is near the outer layer, the converted shaft reading is going to start from the wrong speed. Put the measured winding diameter and the reduction beside the original log.
Separate the speed rating from its operating notes
I would copy the speed row and its notes together. Start with the exact motor code. In one box write the rated condition and in another write any permitted intermittent condition. If the second box has a time limit or applies only at a certain displacement, that belongs beside its rpm figure. A note saying “brief peak” on a work order can't substitute for the conditions in that box.
The Danfoss H1F fixed-displacement bent-axis documentation is a useful example. Its operating limits discussion distinguishes rated speed from maximum speed and states that rated speed is the limit for open circuit applications. The range between rated and maximum speed is subject to the manufacturer's intermittent operation conditions. The document also discusses speed above rated in the context of downhill braking under specified conditions. Those notes are part of the rating. Copying only the largest number from the table would miss the distinction. These are H1F-specific conditions. Use the corresponding data for the motor on the machine. Danfoss H1F technical information
Imagine the printout on the bench gives 1,800 rpm under continuous and 2,200 in a box labelled with a special duty. There is no question about which side of 1,800 our 2,100 falls. What I'm looking for is the note under the box. Is it for the circuit in the machine? What else has to be true about pressure, time and displacement for it to apply? If the supplier sent only the first page, ask for the page with that note. The two rpm limits here are made up for the worksheet.
If there is a peak at the end of all 30 cycles, put that count in the report. A single event during one descent goes on a separate line. Add the time at speed. Where the motor sheet gives a percentage of operating time, get the interval the manufacturer uses for that percentage. Without it there is no denominator for the comparison.
When the flow calculation belongs in the review
I'd start the flow check with 90 L/min at the motor and put a small 1,000 above it to remind me to convert litres to cubic centimetres. Then the two lines are 90,000 ÷ 60 and 90,000 ÷ 45. The first gives 1,500 rpm, the second 2,000. Mark the lines ideal. If 90 was measured back at the pump, find out how much got to the motor before treating the calculation as a useful speed prediction. Check for another branch, flow across a valve and internal leakage.
What changed in this paper example
Ideal rpm
What the value can tell you
90 L/min at 60 cm³/rev
1,500
First estimate for the larger displacement
90 L/min at 45 cm³/rev
2,000
Smaller displacement raises ideal speed at the same flow
75 L/min at 60 cm³/rev
1,250
A flow change alone can also change the estimated speed
Picture the two curves on the same time axis. At the point where flow goes up and displacement stays put, the estimated speed ought to go up as well. Now look at a trace where the pump command holds steady but the loaded machine picks up speed on a grade. The command has very little to say about that part of the plot. I'd follow the torque through the driveline at the instant the machine starts to gain speed.
For a variable motor, ask how the displacement value was obtained. Some service screens show a command, some show feedback and some infer a state from another signal. Give those three cases separate labels. A command for larger displacement does not establish that the rotating group moved there. If the speed changes before the feedback does, the interval is worth identifying in the record. If no feedback is available, record what the drawing and service procedure can establish and leave the actual state open.
The H1B book gives operating information by displacement and discusses the low end of the stroke for certain controls. Lay it beside a displacement trace to see how the comparison works. Which part of the chart corresponds to the time when speed peaked? If the motor carries another code, get its chart before marking a limit. Danfoss H1B technical information
Which side is doing work during the speed rise?
Put an arrow beside the shaft showing which way it was turning at the peak. On the circuit drawing mark A and B as they are installed. A closed-loop transmission can change the sign of the pressure difference across those ports during braking while the rotation arrow stays put. That's a reason to save both pressures with the direction. I'd also put a tick against any displacement change on the same part of the trace.
On the job sheet I would ask for the two work-port channels on the same time base as speed and command, with the machine maker's approved method. A gauge at the pump can be perfectly accurate and still leave the other motor port a mystery. Look at the descent just before the machine picks up speed. Which port is high there, and does it change as the machine slows? Take the labels back to the installed schematic. If a pressure trace has been named for the wrong port, the conclusion can be backwards.
Observation at the event
Next evidence to collect
Speed rises with more measured flow and unchanged displacement
Compare actual flow and speed with the motor's driven operating region
Speed rises as the motor moves to a smaller displacement
Record the verified displacement state and use its speed limit
Speed rises during descent or deceleration while the pump command stays steady
Check load direction, both work-port pressures and the documented braking arrangement
A speed reading jumps with no corresponding motion or pressure change
Verify sensor scaling, signal source and the time reference
Only the display speed rises after a range change
Confirm the engaged ratio and the display's speed reference
The interesting fault is a jump that appears on the display and nowhere else. Pull up the original speed signal if the approved procedure gives you access to it. A second device that logs at one point per second may have been between samples when a short event occurred. I would want the sampling interval written next to each plot before comparing their peaks. Save the raw file if the display applies an extra filter.
Draw the braking path for the installed circuit
On a closed-loop drawing, start at the motor and follow the path when the load drives it. In braking the path may lead back through the pump to the prime mover. Put the pump and prime mover limits on that sketch as well. Danfoss gives examples of downhill and braking operation in its integrated speed limitation documentation for particular H1 pump configurations. Ask which equipment control and fault conditions apply to the machine on the job card. Danfoss H1 integrated speed limitation documentation
An open circuit gives the reviewer a different drawing. A load-control arrangement may meter the outlet side and provide the required inlet support under the specified conditions. A descending load can otherwise drive the motor ahead of the incoming oil. Danfoss's open circuit counterbalance-valve information for H1B motors describes overspeed and inlet cavitation as conditions to address in the circuit design. The equipment maker needs to establish the applicable valve, supply and control strategy, along with the system's fault response. A generic valve recommendation cannot close that design review. Danfoss H1B open circuit counterbalance valve technical information
I'd want three lines on the circuit review. Which part meters or controls motion when the load drives the shaft? Where does the inlet get oil at that moment? What happens if the command or its supply disappears? A valve name alone doesn't answer those questions. Mark the oil paths for the event and send them to the machine maker for review.
If the machine has a parking brake, mark the point in the sequence at which it is allowed to hold or apply. A brake intended to hold at rest cannot be assigned a moving-stop duty from its holding torque figure alone. The equipment owner needs the approved braking strategy and the brake's duty documentation. Any test under load, modification of a protective setting or change to a braking circuit belongs with qualified personnel and the machine maker's procedure.
A record for the peak, not just a maximum rpm value
A job file may have “2,100 rpm on descent” in its first line. If the run lasted two seconds, I'd want to see whether 2,100 was the actual peak, a sample close to it, or a number derived from the wheel or drum. Then count the time above the rated condition from the trace. Put the exact catalog line beside it. The peak without that context will be hard to use on the next machine with a similar complaint.
Field on the record
Detail that makes it usable
What the machine was doing
Start, steady operation, descent, stopping, fault or another named event
Speed source
Motor sensor, independent instrument or conversion from a known output and ratio
Duration
Time above the applicable limit or rated condition and event frequency
Hydraulic state
Work-port readings, verified displacement if variable, fluid temperature and relevant supply condition
Machine configuration
Motor code, transmission range, control settings as approved, and braking components on the installed schematic
Rating evidence
Data-sheet title, revision, model row and the notes that apply to the observed state
Say the pressure file starts at the command and the speed recorder starts at first movement. A reading labelled 0.4 seconds in both files can describe two different moments. I'd put the common trigger on the chart, or leave a note saying that there isn't one. There's a similar trap with a recorder that publishes a one-second average. Its plotted point covers an interval. You can report that interval, but two cursors on the average are a poor way to tell how many milliseconds the shaft actually spent above the line.
Keep an eye on what the logging software does before it draws the line. Imagine the file has values at 0.1-second intervals, with just the 0.5-second point above the limit and both neighbours below it. The record gives you the sample where it was high. It doesn't give the two exact crossing times. A useful note for the reviewer is “one sample above the limit, crossings unresolved at this sampling interval.” Ask for the original file with the sampling and filtering notes if the decision depends on the time.
The pickup bracket is worth a line on the repair record. Say the old one faced the gearbox output and the new one faces the motor. With the original conversion still active, the two installations will give the screen different numbers even if the machine is moving at the same speed. If the only item changed was the motor, check the new displacement and the target the sensor actually sees.
The counter export and the pickup sheet
Field in this example
Value from the example job card
Counter export
1,200 pulses during a one-second interval
Pickup documentation
60 pulses for each revolution of the motor shaft
Old controller setup
30 pulses per revolution
Calculated with the pickup sheet
1,200 rpm
Calculated with the old setup
2,400 rpm
Independent reading at the reducer output
240 rpm, equivalent to 1,200 at the motor with a verified 5:1 ratio
Keep the export in the job file. The first calculation divides 1,200 by 60, giving 20 turns in that second. Multiply by 60 for the minute. On the old setup the division gives 40. If the installer finds the two manuals use different conventions for pulses and edges, have that settled against the equipment instructions. It would be easy to type 60 into a screen and still have the wrong count at the controller input.
Take two points from the file if the machine maker's approved method provides them. I'd use one from steady travel and one from the ramp. A scale error ought to show up in the steady comparison. A lag seen only on the ramp gives the shop a reason to check the averaging and reporting intervals. Leave any actual change to a protective setting with the equipment procedure. If the independent check confirms a real speed rise, use the verified shaft speed for the motor and gearbox checks.
Decide what can be accepted from a quotation
Give the supplier two motor-shaft speeds to work from. One is the loaded running point. The other is the peak with its event and duration. If the speeds came from the gearbox output, put the conversion and selected ratio on the sheet. If the motor can change displacement, say what evidence you have for its state. Name any interval in which the machine was driving the motor.
The response might say “2,200 rpm max” in the speed field and give a pressure below it. My follow-up would attach the 2,100 rpm point with the pressure on both motor ports and the displacement recorded at that instant. I would ask for the exact row that covers the combination. The supplier may need the duration and the circuit sketch as well. Give that question its own line in the quote review so it can be answered in writing.
The BLINCE A2F axial piston motor page gives the buyer a place to identify the family. If the machine plate gives another architecture, start with the BLINCE hydraulic motor range. Put the confirmed order code beside the trace before sending either inquiry. The limit goes into the file when it comes back for the actual unit and its duty.
A question to close before the order goes out
The working question is small enough to fit in an email: does this exact motor code cover the attached event in this circuit? Put the continuous running point beside the event. If the answer uses a separate maximum, ask for its time rule, its displacement range and any pressure condition. Use the reply with its drawing number. Without that number, a future buyer may carry the approval over to the next shaft option or frame size.
I would leave the service ticket open if the conveyor itself went beyond its design speed. In our example the old ticket starts with 330 at the reducer and the event is 420. If 330 was also the equipment limit, check the machine drawing even after the buyer has a motor approval. The equipment maker needs the sketch and the time when the drive stopped holding the intended speed.
Consider two versions of the 2,100 rpm example. In one, the machine is descending, the command hasn't moved and the load takes over the drive. In the other, the machine is on level ground and speed rises just after the displacement changes. On the first printout I would circle the braking interval and the two work-port traces. On the second I'd circle displacement feedback and motor-branch flow. Both need the limit in the catalog. The circles show why the same peak can send the shop to two different places.
A second actuator can make the pump flow log awkward to read. Put the valve state for that branch under the motor-speed trace. In the earlier example the total at the pump is 90 L/min. With 15 going elsewhere, the most left for the motor in that simplified drawing is 75. When that branch shuts, check where the flow goes next. The motor could be receiving more oil while the pump record still shows 90. If the branch feeds a cylinder whose stroke just finished, mark that instant and see if it lines up with the speed change. Check the pressure and return path before assigning the full difference to the motor.
I'd use three boxes on the displacement record. One is the command, one is feedback and the third is whatever the machine maker can verify about actual position. A plot can show a 60 cm³/rev command and a signal nearer 45. That is enough to ask what the feedback represents. If it really is position, go back to the 1,500 and 2,000 rpm rows in the example. If the displayed value is inferred from the command, it gives you little more than the command itself.
Questions that often arise in a speed review
Does hydraulic motor overspeed always mean excess pump flow?
More flow is one answer. A smaller displacement can raise the speed with the pump flow held steady, and a load can turn the shaft on a grade. In the log I'd look for the moment one of those things happened, then see whether the speed moved with it.
Can a motor be run at its maximum listed speed every cycle?
Use the exact rating and its notes. A listed maximum may be reserved for a specified intermittent condition or circuit. The acceptable duration, frequency, pressure and displacement must fit the documented duty for the proposed unit.
Which speed belongs in a comparison if there is a gearbox?
On the sheet above, 420 belongs next to the output sensor and 2,100 beside the motor. Copy the actual range and the engaged ratio across with it so the calculation can be checked.
Which readings should the shop send first?
Send the actual speed trace, the speed reference and the event that produced the peak. Add the full motor code, both work-port pressures where available, and the displacement state for a variable motor. Include the oil temperature and the circuit drawing. That lets the supplier compare the record with a specific rating rather than a product-family maximum.
I'd put “420 rpm, gearbox output, range two” on the ticket even if the ratio drawing is missing. That leaves a usable record for the shop. The calculated box can wait until someone has matched the actual gearbox build and range to the correct drawing.
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
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