100% Employee Owned, Founded 1954


100% Employee Owned, Founded 1954


100% Employee Owned, Founded 1954


Oval Gear Meters – Trouble Shooting Guide – Mechanical Meters

Bill Michie | March 29th, 2017

Flomec Oval Gear meters are designed with durability and accuracy at their core. When used to measure clean lubricating fluids and maintained properly, Flomec Oval Gear Meters will run trouble free for many years. OM Series Mechanical Oval Gear Flow Meters have been the meter of choice for so many and for so long, that the engineers at Flomec have put together some guidelines to ensure that your meter maintains the same optimum performance in the field that it had as a new meter, fresh out of the factory carton.

Correct Orientation

The most basic rule is that of correct positioning. Contrary to popular belief, Oval Gear Flow Meters MUST be mounted so that the rotor shafts are in a horizontal plane.

Failure to mount your Oval Gear meter in the correct orientation (see figure 5-2 below) will cause the weight of the rotors to bear down on the thrust bearings and on the floor of the measuring chamber. Short term effects of incorrect mounting orientation will be a loss of accuracy. Long term effects range from a reduced lifespan (for very small meters) to significant damage to the bearings, rotors, and measuring chamber (for the largest meters). Liquids can flow in a horizontal direction, or a vertical (up) direction, but in each case the rotors shafts must be in a horizontal plane. This is achieved by mounting the meter so that the mechanical display is facing in a horizontal direction as shown on the two meters labeled CORRECT in the image shown below.

One quick note regarding vertical flow installations. To help assist in the elimination of air and/or entrained gases in the fluid, it is recommended that all vertical installations be installed such that the fluid flow is up, rather than down.

All mechanical meters have a flow direction indicator on the body of the meter to assist with flow orientation.

Disassembly and Re-assembly of Oval Gear Mechanical Meters

Mechanical meters are similar in design to that of a pulse meter in that they have the same measuring chamber and use two (2) rotors to measure fluid, but that is it. A Mechanical meter is split into two sections, a wetted and dry section (see below diagram for further explanation). Diagnosing a problem(s) in the wetted section in a mechanical meter is the same as that of a pulse meter, however to gain access to the mechanical measuring chamber, you’ll need to remove the register assembly first.

  • Removing the register assembly requires you to disassemble it first; This is done by removing the upper housing screws from the front of the register assembly and carefully removing the front casing and the gasket.
  • Carefully lift out the mechanical counter display and store to one side. Once done, access to the lower housing screws should now be available; These screws need to be removed so that the lower housing can also be removed. Note, when removing this part, please first make sure the system is off and depressurized.
  • Carefully lift off the register plate assembly and seals. This will give you access to the gearbox assembly.
  • Once the register plate is removed, it is now possible to inspect the reduction gearbox inside the mechanical meter cap assembly. All reduction compound gears should be intact with no visible wear or damage.
  • All mechanical meters have the meter body screws on the same side as the gearbox.
  • It is advisable to remove the flow meter from the installation and relocate to a clean workshop when disassembling a small capacity meter. Once the 8 screws of the meter cap can be removed from the meter body, remove body screws from the top side of the meter cap to access to the wetted section of the flow meter. Care should be taken not to damage the mounting surfaces of the meter body and meter cap. It may be necessary to pry the meter cap off the dowel pins/rotor shafts as they are recessed slightly in the meter cap for larger meters.
  • Take care that the meter body O-Ring and the rotors are not dropped when the meter cap is removed.
  • After disassembly of the flow meter; the O-Ring and rotors should be inspected for any damage, chemical attack, deformity, or any form of deterioration.
  • The bearings should not be loose on the shafts, and the shafts should not be loose in the meter body.
  • Rotor teeth and end faces should have no signs of scoring or wear. By spinning an individual rotor within its measuring chamber by hand (without the other rotor installed) you should observe completely free spinning, without any fouling or interference with the meter body.
  • The first step when re-assembling a “LITERS” meter is to slowly rotate the register plate, this allows the o-ring to be gently worked into the gearbox chamber without causing damage to the seal and also ensures that the register plate mates smoothly with the final reduction gear. This should slide in easily, if hard resistance is met, do not force it in as this causes damage to the gearing, remove the plate slightly and rotate the plate slightly and try again, it should have some flay (about 1 degree back and forth)
  • When re-assembling a “GALLONS” meter, an extra step is required due to the difference between a liters plate and gallons plate. It is important that when inserting the register plate, that the second set of teeth on the Gallons register plate does not come into contact with the gears of that in the gearbox assembly.
  • Re-fit the lower housing casing and apply the 4 screws in a star configuration. Install the mechanical counter carefully ensuring that the horizontal input shaft and bevel gear are correctly located to engage with the smaller bevel gear on top of the register plate.
  • Once fitted, re-insert the gasket followed by the upper housing casing and screws, again in a start configuration

How to Buy a Flow Meter 

Flow meters come in different styles, technologies, abilities, etc. No one meter that can do everything – but they all have their purpose. Before you can buy with confidence, learn what questions must be answered! 

Rotating the Mechanical Register

The mechanical display can be rotated in increments of 90 degrees to suit the orientation of the flow meter in your installation. Note that the flow direction of the flow meter is set by the gearbox. The flow meter must be installed with the correct flow orientation marked by the arrow on the meter, then rotate the display to allow the operator to read the digits.

  • Removing the register assembly requires you to dissassemble it; This is done by removing the upper housing screws from the front of the register assembly and carefully removing the front casing and the gasket.
  • Carefully lift out the mechanical counter display and store to one side. Once that’s done, access to the lower housing screws should be made available. These screws need to be removed so that the lower housing can also be removed. Note… When removing this part, please make sure the system is off and depressurized.
  • Rotate the lower housing casing in the desired orientation and apply the 4 screws in a star configuration
  • Install the mechanical counter carefully ensuring that the horizontal input shaft and bevel gear are correctly located to engage with the smaller bevel gear on top of the register plate.
  • Once fitted, re-insert the gasket followed by the upper housing casing and screws, again in a start configuration.

Reverse Flow

When installing a mechanical oval gear meter, it’s important to take note of the flow direction stamp on the body of the meter and then ensure that the meter has been installed in the correct flow direction. Installation of a meter in the opposite direction will cause the mechanical counter to count down, rather than up. If left to run in a backwards direction for an extended period of time, the resulting damage to the counter will cause it to seize.

In some installations, the meter may inadvertently be installed backwards (flow direction) to accommodate the operator’s view of the register. To correct this condition (or avoid it all together), it’s important to note that the register can be rotated in 90 degree increments to suit the operator’s view. See above for instructions on how to rotate the register.

Noisy Meters

Oval Gear meters are typically designed with just two moving parts. Under normal circumstances, those gears will emit a revving sound during operation of the meter and that sound can vary based on the rotor material and size of the flow meter. If that sound increases over a period of time, it’s very likely that small amounts of debris/foreign particles have built up in the process lines leading to the flow meter and they in turn create a rattling sound inside the meter’s measuring chamber. The obvious solution to this problem is to clean the filtration system leading up to the flow meter to ensure the cleanliness of the fluid as it enters the flow meter. It’s also highly recommended to inspect the measuring chamber and rotors for any foreign debris and resulting damage. If the rotors are damaged and in need of replacement, please refer to the earlier section on “Disassembly” to review how to remove the rotors from the meter.

  • Should excessive wear be found on the rotors in the form of chips in the gearing or score marks etched across the top and bottom of the rotors, then it’s recommended that you replace the rotors to ensure the integrity and accuracy of the meter. It’s also important to ensure that no further damage is done to the walls of the measuring chamber. If excessive damage is found in the measuring chamber, this could result in noise and/or accuracy issues. In most instances, damage to the measuring chamber will require replacement of the flow meter.
  • Cracks or bearing failure within the rotor can result in abnormal movement and ultimately, can cause the rotor(s) to hit the wall of the measuring chamber. Upon inspection, there should be no lateral movement of the rotor in relation to the rotor shaft. In any of these instances, new rotors will be needed. Note: Rotors and Bearings are purchased as a single item.
  • Generally speaking if noise is an issue due to previous damage or high flow rates, it is recommended to install PPS rotors where the application allows it. Meters will run louder when the meter has no fluid in it and is having large amounts of air rush through it. This is not recommended as this can excessive wear on the bearings and cause them to fail.

No Flow / Low Flow

When a “No Flow” and/or “Low Flow” condition is detected, the first thing to check is to be sure the meter has been installed correctly, as outlined in the Operations Manual. If the flow meter has been applied correctly, with an adequate flow that meets the flow meter’s minimum flow requirements, the next thing to check would be external components within the process that could have an affect on the flow, such as a faulty pump, valve that is not functioning correctly or a blocked/clogged strainer upstream of the meter. If these external components are clear, the next most likely cause would be malfunctioning rotor. Rotors subjected to “dirty” fluids can seize if foreign particles are caught up between the individual gears. This condition can also damage the rotor bearings which will eventually also cause them to seize. Damage to the rotors and/or bearings necessitates an inspection of the measuring chamber for any resulting damage that may cause noise and/or damage to replacement rotors. Please be sure to refer to the Operations Manual or the “Disassembly/Reassembly” section found above for instructions. Other causes resulting in jammed and/or damaged rotors include:

  • Chemical incompatibility (this happens more with PPS rotors than Stainless Steel). If you think this may be the case, please review the Flomec/GPI Chemical Compatibility charts to be sure your fluid is compatible with all of the wetted materials included within your flow meter.
  • Rapid temperature changes causing the rotors to expand before the meter body can adjust.
  • Rotor pins distorting due to over pressure causing the rotors to shift their angle inside the measuring chamber and seize.
  • Process fluids drying/curing.
  • Bearing failure due to over speeding which is caused when the flowrate is higher than the meters tolerances allow. Bearing failure can also be a result of air not being bled from the piping on installation or at anytime where the piping is drained between meter use.

Register Issues

Issues with mechanical registers are rare, but not unheard of. They typically come as a result of maintenance or other tampering once the register assembly has been opened. Typical register issues include:

The gasket seal can be inserted upside down. This seal includes a small cutout located at bottom, center that allows entrained air and/or other unwanted fluids to exit the register casing. If reinstalled incorrectly, the register assembly will hold any unwanted fluids and/or air inside it, especially if the installation is outside and/or in an extremely dusty environment. It’s important to note that this will not cause any mechanical issues for the meter, but can make it difficult for the operator to read the display.

Opaque screen. In some applications, the polycarbonate lens on the screen can become distorted making it difficult to take a reading. This is particularly noticeable measuring fluids like Ethanol, Turpentine, etc.. Should this happen, a new screen will be required and appropriate actions will need to taken to prevent this from happening again in the future.

Register numbering. A common fault that occurs with both M3 and M4 registers over time is that the number dials either stick to one another. To resolve this issue:

  • Remove the upper housing screws and casing from the meter.
  • Carefully remove the counter assembly from the housing and place it on a proper work surface.
  • Begin by sliding all of the lower value digits to the left (they are spring loaded). Beginning with the first digit on the right side and working individually, rotate to the desired position and then slide it all the way to the right. Do this for each dial working from right to left.
  • Once completed, re-insert the counter assembly into the housing, ensuring the gearing is seated properly. Reattach the upper housing case and screws.

Register Plate Note

Should any leaking occur from the area between the register assembly and the meter itself, resulting in the fluid entering the register assembly or leaking outside the meter, a new register plate will be required.

When ordering a replacement register plate, it is important to know the materials of construction on your meter. If unsure, your original model number includes keys to each of the materials required for proper replacement of the register plate. Specifically, the 6th and 9th values of the meter code. For instance, for M/N: OM025A001-810, A = Body Material: Aluminum and 1 = O-Ring Material: Viton. Should the register plate need to be replaced, please refer to the Operations Manual that came with your meter for instructions or the “Disassembly/Reassembly” section found above for instructions.

Interested in learning more? Contact a Cross instrumentation specialist to discuss your specific application!

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