Re: Conveyor Drive Gearbox Rating
Dear Angus,
There are too many details you have not noted that can contribute to a superior or inferior reducer selection or other detrimental design/operating factor.
I have witnessed a named WA reducer supplier that walked away from a set of failures on equipment on the scale of 500-800 kW in Australia not a part of this project. Does this mean it could be the suppplier? Definitely. Is the failure due to spaulling? Due to lose of lubricant in the failure zone?
One specific ugly point is your 10 step WR resistor selection. I can guarantee this sequence will yield a start pulse greater than 1.4 times the motor nameplate. I have designed many WR drives and I am an expert on binary stack selection procedures. I note that you have not used the binary stack system. Even with a six step binary sequence (2^6-1 = 63 steps), it is not easy to control the peak torque to less than 1.4 times motor nameplate torque with the best pick of the litter. Whoever tells you otherwise does not know their business. So, although 10 steps were selected, they still may yield a torque pulse much greater than you expect or measure.
What is your selection of permanent impedance with respect to percentage of motor slip?
How do you know that your DAQ sampling rate is adequate or that the DAQ signal is not muted with a unacceptable signal filter to make it look good?
Still, many drives with this rude toque pulses should not damage a reducer with a mechanical SF=1.5. The SF=1.5 is also not a telling number. Be more specific and get the tooth and endurance strength rating with respect to the motor nameplate. These two values are or should be greater than 1.5xnmpl. CDI has provided Hammersly Iron with proper reducer specifications.
Another factor for right angle boxes is the Reed Frequency resonant mode. Do you know what it is for this box? Did the manufacturer do the test or calculation? We had a 3000kW box go bad from a bad gear pitch selection where it set up resonance in the box.
A conveyor can cause destructive resonance due to operating in the push-pull action near zero power. Is the conveyor operating in a near neutral power case? What about the torque arm? How was its position selected wrt the range of power/torque.
There are too many questions that could be asked but does not serve the purpose of this forum.
If you wish my help, you know how to contact me. ■
Re: Conveyor Drive Gearbox Rating
What was the ambient temperature used for determining the thermal ratings of the drives?
How was the peak intermittent load spedified? This is used as a criteria for strength rather than fatigue.
What is the project?
Who is the supplier? ■
Re: Conveyor Drive Gearbox Rating
Can any one suggest a peak intermittent torque as a percent of full load torque at which a drive should be rated for 'strength'. I'm writing a specification at the moment.
My recollection is that this should typically be in the order of 200 % to 250 % of motor full load torque even with a soft start. This may be excessive if you could convince yourself that the 140 to 150 % start factor will never be exceeded on site.
This may also be relevant to the OP. ■
Conveyor Drive Gearbox rating
I would be interested in the selection parameters for gearboxes and holdbacks using in on conveyors, particularly mechanical service factors. We have had a significant number of failures with one particular project and are concerned on the results of the selection process for the gearboxes and holdbacks.
The conveyors we use are:
Width: 1200 to 1800 mm
Length: 50 m to 1000 m
Speed: 3 to 5 m/s
Material: Iron Ore 3,000 to 10,000 tph
Motor Power: 300 to 900 kw (multiple drives are used)
Number of starts: Varies with location could be as high as 10 - 20 per day.
The Conveyor Drives consist of a 90 degree gearbox (two stage reduction) which is directly connected to the conveyor drive pulley by a rigid coupling and the motor and brake is supported on a drive base attached to the gearbox (torque arm arrangement). The motors are wound rotor type with a stage resistance start which is usually about 10 steps with a design maximum of 1.4PU.
The brakes are a disk type mounted on the gearbox high speed shafts.
The hold backs are a centrifugal lift off type mounted on the side of the gearbox by the use of a longer intermediate shaft.
The conveyor designers are experienced with their methods (static) and the calculated demand and the field measurements are close hence the initial calculated drive power is close to the actual.
We have conducted strain gauging, high speed logging to attempt to identify any particular dynamic effects, of which none were identified.
The failure analysis on the gearboxes does not show any obvious defects with the manufacture. The mode of failure is gear tooth failure.
The failure analysis on the holdbacks by the manufacture indicated “popping” as the likely cause of their catastrophic failures.
The selection method adopted at the time was on the gearbox vendors recommendations, which was for a service factor of 1.5 on the demand power being less than the catalogue rating of the gearbox. The change to traditional methods was that the successful vender specifies and this was accepted a high thermal rating due to a separate electrically driven fan. Hence the mechanical rating was the limitation of the gearbox not thermal which has been our previous experience.
Questions:
1.In the experience of other members of this forum, for the conveyors described above does the selection method appear reasonable.
2.What alternative selection methods would be recommended for this starting type. ■