Hey there, fellow hydraulics folks! Let’s cut to the chase: if you’re working with Rexroth hydraulic motors, you know how crucial accurate torque measurement is. Whether you’re troubleshooting a field unit, qualifying a rebuilt motor for your shop, or fine-tuning a custom machine build, guessing torque isn’t just lazy—it can lead to wasted energy, premature part failure, or even a botched job that costs you clients. As a Rexroth hydraulic motor supplier who’s been knee-deep in this gear (pun totally intended) for over a decade, I’ve seen so many people skip the right steps and end up with results that are off by 15% or more. That’s why today, I’m breaking down exactly how to nail accurate torque measurement for Rexroth motors—no fancy, overpriced tools required if you do it smart. Rexroth Hydraulic Motor
First, let’s get clear on what we’re actually measuring here. Torque from a hydraulic motor isn’t just a number; it’s the twisting force the motor outputs to turn your load. For Rexroth, this is tied directly to their design specs: most of their bent-axis or axial piston motors (the ones everyone uses for mobile equipment, industrial presses, and material handling) have a formula for theoretical torque, but real-world output is where the rubber meets the road. Theoretical torque is easy to calculate (we’ll cover that), but accurate measurement accounts for losses from friction, internal leakage, and pressure drops—stuff that the manual just gives you as a baseline.
First rule: you can’t measure torque right if your test setup is garbage. I’ve had customers bring me motors they tested on a cheap test bench with wobbly mounts, and their results were useless. For Rexroth motors specifically, you need a setup that matches their mounting dimensions—Rexroth uses standard SAE mounts for most of their popular models (like the A10VSO series), so skip the one-size-fits-all adapter plate; get a mount that bolts directly to your motor’s output flange and the torque transducer, no slop allowed. Slop = inaccurate readings, full stop.
Next, pick the right torque measurement tool. There are two main types people use: reaction torque transducers and inline torque sensors. Let’s keep this simple: if you’re testing a motor in a static or low-speed application (like a test bench), a reaction torque transducer is your best bet. If you’re testing a motor under actual load while it’s installed on a machine, an inline sensor works, but that’s way more niche. For day-to-day use in a shop, reaction torque transducers are the way to go—they’re affordable, easy to calibrate, and perfect for Rexroth motors because they mount to the motor’s housing (not the output shaft), so you don’t have to deal with shaft alignment issues. Pro tip: make sure the transducer’s range is 10-110% of the motor’s rated torque. If your Rexroth A10VSO has a 200 Nm rating, don’t use a transducer that maxes out at 500 Nm—you’ll miss the small, precise changes that matter.
Now, let’s talk test conditions—this is where most people mess up. Rexroth motors don’t perform the same in every environment, so your test has to match the conditions the motor will actually see. If you’re testing a mobile motor for a dump truck, don’t test it at 100°F if the truck will run in 10°F winter weather. Hydraulic fluid viscosity changes with temperature, and that kills torque accuracy. For Rexroth, always use the fluid type they specify (usually ISO VG 46 or 68) and get the fluid temperature to the operating temp before you test—120-140°F is standard for most industrial uses, but check your Rexroth manual for your specific model. Also, control the inlet pressure and outlet pressure (back pressure) exactly. Rexroth’s torque formula is based on pressure delta: inlet pressure minus outlet pressure. If your outlet pressure is 50 psi higher than it should be, your torque will be off by that delta’s effect—trust me, I’ve spent hours chasing a leak in a line only to realize I forgot to cap the outlet port and let air in, which made back pressure random.
Wait, let’s circle back to that theoretical torque formula—because it’s a great baseline, and a quick way to check if your test readings make sense. The formula for Rexroth axial piston motors is: T = (P * V / 2π) * ηm, where T is torque in Nm, P is pressure delta (psi or bar), V is displacement in cc/rev, and ηm is mechanical efficiency. Rexroth lists displacement right on the motor’s nameplate—no digging required. So if your test reads 180 Nm and the formula says 210 Nm, that tells you your motor’s efficiency is a bit low, which is a red flag for internal wear. But don’t rely on the formula alone; real measurement beats theory every time.
Now, step-by-step for accurate measurement, using a Rexroth motor as our example. Let’s say we’re testing a rebuilt Rexroth A10VSO 28, which has a displacement of 28 cc/rev, rated torque of 105 Nm at 250 bar delta. Here’s how to do it right:
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Prep the test rig: Mount your Rexroth A10VSO on a rigid SAE A mount (match the motor’s spec—do not use a smaller or larger SAE mount, I’ve seen someone bend a flange trying that). Bolt the reaction torque transducer directly to the motor’s housing, and connect the transducer’s output cable to a digital display (avoid analog gauges—they’re too hard to read precise changes). Connect the motor’s inlet to a variable pressure hydraulic pump, and the outlet to a load valve (to adjust back pressure). Fill the system with the correct ISO VG 46 fluid, and let it run for 10 minutes at low speed (500 RPM) to get to 130°F.
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Set your test parameters: Close the load valve to get back pressure to 0 psi (unless your application has a specific back pressure, then use that). Increase inlet pressure slowly until you hit a delta of 100 bar, then 150, then 200, then 250. For each pressure step, wait 30 seconds for the system to stabilize—don’t take a reading right after adjusting pressure, because pressure spikes will skew the torque. For the A10VSO 28, at 250 bar delta, you should see around 95-102 Nm of torque (real-world efficiency is usually 90-97% for a good Rexroth motor, which lines up with that). If it’s below 90 Nm, you’ve got internal leakage or wear.
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Check for errors: After each test, disconnect the sensor and run a quick no-load test—spin the motor by hand with no pressure. If the torque reads more than 5 Nm, your mount is loose or there’s friction in the transducer setup. Another check: reverse the motor’s flow direction (inlet becomes outlet) and repeat the test. If the torque is significantly different, there’s an internal imbalance in the motor, which is a problem specific to Rexroth motors if it’s a new one—shouldn’t happen, but it’s common in rebuilt units that were machined wrong.
Wait, what if you can’t take the motor out of service to test it? That’s the most common scenario I hear from customers. If you need to measure torque on a motor that’s already installed on a machine (like a conveyor drive or a winch), you can’t use a reaction transducer, but there’s a work-around. Use a formula based on the load: measure the speed of the motor’s output shaft with a tachometer, and measure the power the hydraulic pump is drawing (use a clamp-on ammeter if it’s an electric pump). Then use Torque = (Pump Power * Efficiency * 5252) / Motor Speed. But here’s the catch—this method is less accurate, because you have to account for pump and motor efficiency, which can vary. For Rexroth motors, I always tell customers to add a 5% margin of error here, so it’s good for troubleshooting, not for precision jobs.
Another common mistake people make: using a portable torque wrench to twist the output shaft. That’s for static torque, not dynamic operating torque. If your motor is turning at 1000 RPM, the torque is totally different than when it’s stopped. Portable torque wrenches work for checking residual torque on a stalled motor, but don’t use that as your only measurement. I’ve seen a customer swear his motor was fine because the torque wrench read the right number, but when we tested it on the bench, it was 20% low at operating speed. Moral of the story: test at the motor’s operating speed, not stalled.
Now, Rexroth-specific tips. These are things that only a supplier who works with their motors every day would know. First, their bent-axis motors (like the A6VM series) have a slightly different efficiency curve at low speeds—if you’re testing an A6VM at speeds below 500 RPM, you need to adjust for their internal friction, because the formula doesn’t account for that. Second, their servo motors (the MS series) have a much tighter torque tolerance—so you need a transducer that’s accurate to ±0.5%, not ±1%, otherwise you’ll miss tiny deviations that matter for precision applications like robotics. Third, always clean the ports on your Rexroth motor before connecting lines—even a tiny bit of debris in a port can cause internal leakage, which makes torque readings lower than they should be. I once had a customer bring in a new A11VLO motor that read 15% low, and we found a metal shard in the inlet port—cleaned it out, and the torque was right back to spec.
What about calibration? You can’t just plug in a transducer and use it—you have to calibrate it at least once a year, especially if you’re using it daily. For Rexroth, the good news is that their recommended calibration is done with a traceable standard (like a known weight on a lever arm) to make sure your readings are accurate. A lot of small shops skip calibration, which is why their torque numbers are all over the place. As a supplier, we always calibrate our test benches before we ship a rebuilt Rexroth motor, and we tell customers to do the same for their setups.
Now, what do you do with the measurements once you have them? For example, if your test on a Rexroth A10VSO shows torque is 10% below spec, that means you have internal leakage—could be worn piston shoes, a bad seal, or a cracked valve plate. If it’s 5% low, that’s usually just normal wear for a motor that’s been running 5000+ hours. If it’s over spec, that means your pressure delta is higher than you thought, which could mean a clog in the outlet line or a bad pressure gauge. The key is to cross-reference your readings with Rexroth’s performance curves for your specific model—they’re available on their website, but as a supplier, we keep a binder of them in our shop so we can match up readings fast.
Let’s wrap this up with the most important part: if you’re not comfortable doing this yourself, or you need to test a critical Rexroth motor for a big job, don’t guess. As a Rexroth hydraulic motor supplier, we offer on-site testing and bench testing services for all their models, and we calibrate every tool to Rexroth’s exact specs. I’ve seen customers ruin thousands of dollars in equipment because they tested a motor wrong, so don’t risk it. If you need to measure torque accurately, or you’re looking for a new or rebuilt Rexroth motor that’s already been tested and verified, reach out to us. We’ll send over a team to help you set up the test, or we can handle the whole testing process for you so you know exactly what torque you’re working with.
Wait, let me make sure I didn’t miss anything. Oh—pressure spikes! They’re a huge issue for torque measurement. When you open or close a valve, you get a pressure spike that can make the torque reading jump 20% in a fraction of a second. That’s why you have to wait for the pressure to stabilize—30 seconds minimum for Rexroth motors, because their hydraulic circuits are designed with certain volume capacities. If you rush, you’ll get a spike reading that’s way off. Also, don’t test at maximum pressure for more than 5 minutes if you’re just troubleshooting—Rexroth motors can handle it, but it can cause unnecessary wear if you’re not gathering data.
Another thing: shaft alignment. If you’re using an inline torque sensor, misalignment between the motor and the sensor will cause friction, which adds to the torque reading. For Rexroth motors, their output shafts have a keyway, so make sure the adapter for the sensor is aligned perfectly—use a dial indicator to check, not just eyeball it. I’ve seen a customer eyeball alignment, and the reading was 12% high because of misalignment friction. Cost him a whole day of troubleshooting before he realized it.
Look, I know this stuff can be overwhelming, but once you nail the setup, it’s not that hard. The big takeaways are: match your test setup to Rexroth’s specs, use the right tool, control test conditions (temperature, pressure, speed), calibrate regularly, and don’t guess on the numbers. If you’re in the market for Rexroth motors, or you need help testing yours accurately, don’t hesitate to connect with us. We’ve got the parts, the tools, and the expertise to make sure you get the right torque measurement every time.
Small Hydraulic Winch References
- Bosch Rexroth. (2022). Hydraulic Motor Performance Specifications & Test Procedures. Rexroth Industrial Division.
- Fluid Power Society. (2021). Accurate Torque Measurement for Hydraulic Components. National Fluid Power Association.
- Hansen, J. (2020). Troubleshooting Axial Piston Hydraulic Motors. Hydraulic & Pneumatic Systems Magazine.
- ISO 4413:2019. (2019). Hydraulic fluid power—General rules and safety requirements for systems and their components. International Organization for Standardization.
Jining Jinjia Hydraulic Co., Ltd.
Jining Jinjia Hydraulic Co., Ltd. is one of the most professional rexroth hydraulic motor manufacturers and suppliers in China, specialized in providing high quality orbit hydraulic motors. We warmly welcome you to buy customized rexroth hydraulic motor at competitive price from our factory.
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