Travel Motor
Revision: 8/OCT/2024.
Lets talk about the travel motor responsible of driving your excavator. This motor is called the "final drive" or "track drive". The technical name is a Low Speed High Torque (LSHT) Geroler Orbital Hydraulic Wheel Motor. Let's dissect that, an orbital motor just means it rotates a shaft and a geroler motor uses rollers instead of lobes to produce the rotation. Notice that a geroler is a type of gerotor motor.
The final drive is mainly designed for torque for example in mini excavator an common motor with 20 MPa of hydraulic pressure will be able to produce 810 Nm (597 ft*lb) or more. When they say they are slow they mean it, at that same pressure and flow you will only get about 22 revolutions in a minute. You are literally “crawling” at a blistering speed of about 0.9 mph.
SIDE NOTE Before you try to turn your mini into a speed roadster lets spare you the agony with a pragmatic reality check. Unless you upgrade most of your hydraulic system your machine will be limited to a nominal pressure of 20MPa. At that pressure the only thing you can attempt is to get another pump that can produce a higher flow rate. If you double the flow you almost double the RPM thus your speed so you are now blazing at 2 mph! The problem is that by the time that you try to get a PUMP at 20MPa that can get you at 3 mph you realize that none of the standard pumps have the required flow, that you need to do some fabrication, and good luck finding something that will fit the space you have available. At that time we hope that you realize that even the fancier Caterpillar Mini Excavator will only do 3 mph for a reason… |
The main specifications for a wheel hydraulic motor is the torque and the radial load carrying capacity. Proper calculation of these takes into consideration factors like rolling resistance, largest grade, weight of the machine, etc.
Like we mentioned in other guides the only job of the pump is to provide and sustain a steady flow of hydraulic fluid. It takes a specific volume of hydraulic oil to cause movement (displacement) on a motor. The displacement rating of a motor tells us how much oil it takes to rotate the motor one full revolution. The displacement is measured in cubic centimeters per revolution where 1 cm³ is about 0.000264 gallons. Common wheel motors on mini excavators will have displacements from around 310 cm³/rev to 315 cm³/rev.
The travel motor is able to rotate at a given speed (RPM) with a given force (Torque, N-m) when a particular flow is sustained by the hydraulic pump. For example a traditional mini excavator pump operating at nominal capacity of 20 MPa at 2500 RPM will be sustaining a flow of about 21 Liters per minute. With this flow we will see regular wheel motors producing around 800 N-m of torque and doing about 50 rpm.
Just like everything else in a hydraulic system the travel motor is also designed to handle a continuous level of pressure and some incidental peaks. The continuous pressure is the nominal pressure. For example in the BMT, OMV, OMT, BM6 and 6000 series of motors it is common to see 20MPa as the nominal pressure.
When it comes to identifying a final drive there are a couple of things to look for. Before we continue, let's clarify that we will only cover the more common motors used in Chinese excavators.
| In general we can tell if a geroler motor is a wheel motor because a wheel motor will have the mounting flange close to the center of the motor and not at the end. It is also common for a wheel motor to have a case drain port. |
When looking at a manufacturers catalog we will limit ourselves to “wheel mounts” and lucky for us the offerings for wheel motors are limited, greatly reducing the combinations that we need to look for to match a motor.
The next thing we need to look at is at the mount itself. Wheel motors at the size used on mini will always use 4 screw rectangular flange mounts.
| The main characteristic of the mount is the circle formed by the screws. This is called the Bolt Circle and it is the main measurement used to match a mount. In a 6K motor the diameter of the bolt circle is 184.15 mm. You only need to consider the size of the rectangle just to make sure it fits in the space available. |
The other common bolt circle has a diameter of 200 mm and it is used in OMT, BMT, BM6 and similar motors. The beautiful thing is that once you match the bolt circle you are almost done identifying a final drive motor. The next thing we need is to identify the shaft and hydraulic ports.
The travel motor attaches to the sprocket wheel by the tapered shaft. There are two basic taper shafts in motors. The first has a 1:8 reduction ratio and the second has a 1:10 reduction ratio. The 1:8 reduction is the one you commonly find on mini excavators and it uses a 1-1/4-18 UNEF-2A thread and castle nut. This is commonly named as “1-1/4 Inch Tapered Shaft”. |
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The 1:10 taper is metric with a M30 x 2 castle nut. The 1:8 taper is SAE with a 1 1/4 - 18 UNEF (1.25”/31.75mm nominal diameter, Pitch TPI 18) castle nut.
To save you time we suggest you look at a manufacturer catalog and look at the wheel motors that match the mount and shaft. You will notice that they only offer a very few combinations for a given model. Usually they only have one mount size and will offer two or three shaft types, but only one particular port type is offered per shaft type.
For a 1-¼, 1:8 Tapered shaft the ports are almost always BSP ports that are either G3/4 or G1 with a G1/4 case drain.
The displacement of the wheel motor will always be in function of your hydraulic pump. For example a common 25 MPa pump has a nominal pressure of 20 MPa and a 305 displacement. This means our motor will also have a similar nominal pressure of 20 MPa and a displacement very close to your pump. Lucky for us pumps and wheel motors only come in a given set of displacement values. For example wheel motors come in displacements like 310, 315 and 325, etc.
Aftermarket options
The most common model of final drive used in Chinese mini excavators is a white label Char-Lynn 6000 series wheel motor also known as the 6k Series. The Char-Lynn Hydraulic was a Minnesota company credited with the invention of low-speed high torque motors, they were later acquired by Eaton and now only exists as a brand name within Eaton. In 2021 the Danish company Danfoss acquired Eaton.
There are many clones for the 6000 series, some of them are meant as aftermarket replacements and others are their own variations. Danfoss itself also manufactures the “OM” series of motors which also includes the “OMT” motors which is similar to the 6000 series. White label Chinese manufacturers like HAN-JIU Hydraulics have the “BMT”, “BM6” and “BMK” motors. On the Parker side we have the “TK” series.
The main white label wheel motor made by Chinese manufacturers uses the “BM” designation. The “BM” designation is mainly intended as an aftermarket replacement for Danfoss “OM” motors. For example “BMR”, “BMV”, “BMT” are homologous to “OMR”, “OMV”, “OMT”, etc. These white-label motors also have designations for other popular motors, for example the designation “BMK” and “BM6” are aftermarket replacements for the Char-Lynn 6000 series. The availability and affordability of “BM” motors from Chinese manufacturers make them the de facto motor used in mini excavators.
Older models of mini-excavators used the Char-Lynn 4000 series (or BM5) motors.
When looking for an aftermarket replacement ALWAYS match the mount dimensions as not all BM6 wheel motors will have the same mount. In some instances a BM6 will have a BMT/OMV mount instead of a 6k mount.
Brand | RIPPA | EATON Char-Lynn | HAN JIU | PARKER | EATON Char-Lynn | Char-Lynn | Char-Lynn | HAN JIU |
Designation | 4K??? | 6000 Series | BMK6 | TK | 4000 Series | OMT-W-315 | OMT-W-400 | BMT |
Displacement | 315 | 310 | 315 | 315 | 310 | 315 | 400 | 315 |
Type | Geroler | Geroler | Geroler | Geroler | Geroler | Geroler | Geroler | Geroler |
Mount | AC | W | T | |||||
Screw Circle/Center | 184.1 | 184.15/130.23 | 184.12 | 184.5 | 165/116.74 | 200.4 | 200.4 | 200 |
Flange Rectangle | 167 | 166.9 | 170 | 166.8 | 148 | 180 | 180 | 180 |
L2 | 104 | 115.6 | 115.5 | 163.28 | 135.1 | 94 | 104 | 91 |
Staggered Ports | G1 | G1 | G3/4 | G1/2 | G3/4 | G3/4 | G3/4 | |
Torque (Nm) | 930 | 920 | 970 | 950 | 1080 | |||
Pressure Inlet Bar | 205 | 20MPa | 200 | 210 | 210 | |||
Tapered Shaft (in) | 1.75 | 1.75 | 1.25 | 1.75 | 1.75 | 1.125 |
Troubleshooting
Unusual grinding noises from your final drive should not be ignored. A common source of noise is insufficient oil levels, check for leaks and your hydraulic oil level. Impurities in the oil like metal swarf may be causing friction, flush your hydraulic system, replace the return oil filter and change your hydraulic oil. When flushing a final drive remember to open the case drain plug as most impurities will accumulate in the case drain cavity. Remember that you will not be able to flush all impurities from the drive and you also may have worn out parts. We suggest that you disassemble the motor, clean it up and check the integrity of the parts.
A drive motor that is getting too hot is more likely caused by either low oil levels or worn bearings causing friction.
Oil leaks are a sign of worn or broken seals. You should be able to source a seal repair kit online. You can also contact us as we keep an inventory of seal kits for the machines we sell. Remember high pressures caused by an improper adjustment of your DVC Pressure Valve can rapidly deteriorate the seal of motors and cylinders. This is also the case if you let your hydraulic oil get too hot.
If your oil level and flow is good and you have low performance or travel from your drive it could be that your track tension is too tight, an indication of an obstruction of fluid like a clogged case drain or a more serious issue with the gearbox or shaft.
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