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1. How is precision of the screw rod feeder correctly described?
Based on the standard variance sum average principle of mathematical statistics，we usually use repeatable accuracy and linear accuracy to describe accuracy of the screw rod feeder. The repeatable accuracy is an index of keeping consistent with flowrate of the screw rod feeder. The linear precision is an accuracy index for working flowrate scope of the screw rod feeder.

Repeatable accuracy：

Based on standard sample variance, repeatable accuracy describes discrete status for flowrate sample of the screw rod feeder within each of some sampling periods in a period of time and is one of important indices of describing repetitive error of the screw rod feeder. Based on standard variance, repetitive error can be quantified, with its computing formula as follows：

N is sample quantity and x is sample weight in sampling period.

We usually describe repeatable accuracy with 95.5% sample weight within range of 2STDEV(2 sigma)：
Repeatable accuracy % = x average weight / 2STDEV

How to correctly sample to obtain standard variance is key to accurately describing repetitive error of the screw rod feeder. Because a majority of factories are short of professional measuring means，traditional sampling is acquired manually. The tools are stopwatch，two standard containers，electronic scale used for sampling， record paper，test raw material and screw rod feeder to be measured.

Place electronic scale below the screw rod discharge port and a container on electronic scale; get ready for stopwatch below outlet of the screw rod feeder; put test raw material into stock bin of feeder; Start running the screw rod feeder on a certain set flowrate; after stabilization， start stopwatch; The sampling period is generally 1min. When sampling time reaches，read and record weight on electronic scale. Generally take 30 samples; after container is full，it can be replaced by the other container to keep on sampling.

Major factors affecting manual measuring accuracy are the weighing accuracy of electronic scale and timekeeping error of hand sampling. The smaller the measured sample weight, the higher the electronic scale accuracy required. Accuracy is usually 0.1% of sample weight. In order to acquire accurate sampling timekeeping， be sure to appropriately increase sampling timekeeping time and decrease human error of sample weight. Meanwhile it is required that the same person performs timekeeping operation and sample acquisition continually proceeds.

Professional feeder manufacturers usually adopt automatic routine for sampling to ensure precise sampling, to decrease human error of hand sampling and simplify data processing procedure. The tool used by standard automatic routine sampling is the high-precision electronic scale. The electronic scale is connected with a data processing computer through standard communication port, such as RS232 or Ethernet port, so as to use standard data processing program for immediate processing of data to figure out accuracy indices.

Sonner Company adopts a sampling program developed independently to realize differencing dynamic sampling. Such sampling method can randomly set output sample weight in interval from 0.1 second to 10 seconds so as to satisfy demand of sample weight data processing in subsequent 1min, to acquire any group of sample weight without system error and correctly evaluate repeatable accuracy and linear accuracy of feeder.

Linear accuracy
Linear accuracy is to describe accurate degree of each operating point within operation range of feeder from minimal feeding capacity to maximal feeding capacity. It is namely an error between actual feeding capacity and set feeding capacity within whole measurement range. The smaller the error, the higher the linear accuracy of feeder represented.

The representation formula is as follows：

X is the average sample weight, F is the standard setting weight.

In order to obtain linear accuracy, be sure to collect array sample weight in definite time like the method of measuring repeatable accuracy of feeder. Under the general situation，we continually take 4 arrays under 5%，25%，50%，75% and 100% flowrate, with 10 sample weights in each group, and then calculate D-value between average sample weight of each group and setting weights. According to the above formula, the linearity error rate of each set point is figured out. These errors are namely linear accuracy，representing accuracy of feeder within range of its operation.

2. How is accuracy required by the screw rod feeder in requirement of customer''s processing technic confirmed?

To correctly comprehend customer''s processing technic information, product technology specs and quality standard is key to successfully selecting the screw rod feeder.

Customer''s processing technic information includes raw material formula composition，raw material feature，productive capacity scope of whole product line and installation environment at users’ site. The component lectotype for the screw rod feeder is confirmed by analyzing and confirming the above information, such as feeder''s metering screw specs, weighting component accuracy, measurement range and stock bin cubage of raw material, to ensure feeder system composition is optimal and satisfy feeder accuracy requirements of user''s product quality standard.

Owing to physical characteristic complexity of solid raw material in mixed prilling, it makes accuracy represented on the same screw rod feeder different， which requires us to more cautiously select the screw rod feeder. Usually most of professional screw rod feeder manufacturers make much account of various solid raw material tests and confirmed combination of optimal screw rod feeder to satisfy user''s technic requirement.

3. Under what circumstance are volume type and weightless type feeder selected?

The weightless type feeder attains and maintains the set flowrate by detecting actual flowrate and automatically adjusting rotation speed of the feeder screw rod. ，The volume type feeder calibrates out flowrate value corresponding to each point within whole screw rod speed range by sampling raw material to be conveyed before running. If it is replaced by another raw material，be sure to recalibrate，and then according to the calibrated value and flowrate to be operated, conclude out the screw rod rotation speed, under which to run feeder.

The volume type feeder, which is controlled by open loop, is characterized by considerable economy，simple maintenance, low installation environment requirement and other advantages. However when raw material stacking density is larger in change, it will cause the volume type screw rod feeding accuracy to be difficult to control and other shortcomings. Relative to the weightless type feeder, the volume type screw rod feeder is an economic feeding solution. Therefore the volume type screw rod feeder is generally applied in processing technic, accuracy of which is not the most key technology index. The factor affecting accuracy of such scheme includes three aspects: geometrical mechanical structure of the screw rod as metering component，such as combining form of single screw rod and dual screw rod； The screw structure form of screw rod：spring form，Auger form，Concave form；the screw rod feeding speed and raw material physical state，such as stacking density，fluidity，material viscosity.

By understanding raw material status characteristic，selecting adaptive screw rod combination and screw structure form can appropriately improve and ensure accuracy of the volume type feeder. For example：for a majority of graininess raw material，especially for fresh material, we generally select single screw rod volume type feeder. Because such raw material granule is uniform with good fluidity, its stacking density is quite stable and consistent in transport process. The volume type screw rod feeder can likewise attain higher repeatable accuracy and linear accuracy. Only it needs to perform off-line sampling and calibration and other cockamamie operations to accurately attain the set flowrate to be operated. We generally select dual screw rod volume type feeder for powder raw material because a majority of powders are not good in fluidity and larger in viscosity. Although dual screw rod can overcome retention and bridging problems in spiral flute，it cannot ensure raw material cubage is constant in each spiral flute, so accuracy of the volume type screw rod feeder is very poor for such raw material and should be cautious in adoption.

4. What is major factor affecting operation accuracy of the weightless type screw rod feeder in user''s factory?

Correct design

The weightless feeder system is the 1st factor affecting the weightless type feeder. The weightless type feeder includes a flow control system of continuous operation，so it needs a high dynamic, high accuracy weighting system to detect feeder outlet flowrate in a short time so as to ensure flow control accuracy. Be sure to correctly adopt the screw rod and speed range to match raw material physical characteristic and working flowrate scope. Meanwhile be sure to consider the feeding bin cubage and anti-bridging device to optimize system.

In order to ensure the weightless type screw rod feeder work under high accuracy status，be sure to not only correctly evaluate customer''s processing technique and raw material information feature and select suitable the weightless type screw rod feeder, but also to systematically design the raw material feed system, mixed baiting system and auxiliary exhaust system of the weightless screw rod feeder.

Installation and operating environment
This is the 2nd factor affecting accuracy of the weightless type screw rod feeder. We usually should pay attention to several points below in design and installation：

1. An independent separate type design is adopted for the raw material replenishing bin system and weightless feeder platform, which should be firm to ensure mechanical shock wouldn''t affect stable operation of the weightless type feeding system when raw material feed system runs. Independent workspace for maintenance and operating personnel can be designed to ensure walking and operation of operating personnel wouldn''t affect stable operation of the weightless feeding system.

2. The reliable flexible connection device should be designed for raw material outlet, inlet and air vent of the weightless type screw rod feeder so as to insulate influence of mechanical shock on weightless feeding system.

Material replenishing process control

When raw material consumption in weightless feeder bin reaches minimal replenishing amount，system can automatically or manually enter material replenishing process control. At this time, because system weight remains at increase stage, it cannot perform closed loop weight control. The system remains at volume type open-loop control status until replenishing amount attains the set weight. Only after material replenishing stage finishes，will system automatically or manually return to weightless type closed-loop control stage over again. It will be seen from this that any weightless type screw rod feeder cannot avoid volume type running during material replenishing period.

Theoretically，the shorter the volume type running time，the longer the weightless running time, the smaller the influence of volume type running accuracy on weightless running accuracy. But it is really impossible to be realized, so we generally select at least 10∶1 as safe specific value of weightless running time/volume type running time. Certainly, the bigger the specific value, the smaller the accuracy influence caused by replenishing material. Because ingoing raw material destroys barometric balancing in the feeding bin in material replenishing stage, the system exhaust would occur.

If exhaust is not unobstructed，it will likewise prolong material replenishing time, thereby affecting system running accuracy. Besides above factors，because material levels in bin before and after replenishing are different, it will cause certain change in raw material stacking density. In order to decrease unsteadiness time of system from material replenishing stage restoring to weightless type control stage，it is very necessary to optimize the screw rod speed in volume type running during material replenishing period. The optimized screw rod speed is figured out through historical data model. The system accuracy effect is far better than the screw rod speed before keeping material replenishing.

5. What are constant faults? How is maintenance correctly performed?

No matter whether it is volume type or the weightless type screw rod feeder, the key problems existent in run procedure of user''s factory are incorrect installation, non-normal maintenance of equipment， insufficient training for operator and repairman, change in user''s processing technique, raw material and operating status，and too large difference from technical conditions confirmed during ordering.

Main problems and correct maintenance method for the volume type feeder
Because the volume type screw rod feeder is a simple, open loop control unit, it usually consists of the feeding bin，screw rod feeding part，motor drive part and variable frequency speed controller. Such simple machine control unit is quite simple in installation and can adapt to quite hostile environment. So long as you can periodically inspect motor control wiring and periodically clean the screw rod and feeding bin, fault can be avoided basically.

The key problems of the volume type screw rod feeder are the screw rod seizure and bridging problems, main reasons of which come from 2 aspects：

1. Change in user''s raw material has the present screw rod unable to adapt. For example, if the screw rod feeder selected is used for conveying powder raw material，dual screw rod would usually be adopted to ensure stable conveying. However, if it is changed into conveying granular material，most of dual screw rod structure would generate seizure for granular raw material.

2. The large size foreign matters existent in user''s raw material for the reason of supplier，especially in some recycled raw materials, have the screw rod seized.

3. The equipment is not cleaned after shutdown, which have some raw materials agglomerate in the screw rod and feeding bin as a result of retention for a long time. When running again at the next time，the screw rod will be seized. In order to solve above problems, be sure to mainly replace the screw rod to adapt to variance in raw materials, be sure to take effective measure to control raw material cleanliness and perform cleaning in time after shutdown.

Main problems and correct maintenance method for the weightless type feeder:

The weightless type feeder is a quite complicated process control system instead of a simple device. Its FAQ and faults are as follows：

Incorrect installation of the weightless type feeder system is the 1st important reason to have it constantly generate alarm faults and unable to run normally. 1. The platform structure is thin and weak；2. The flexible connection for raw material inlet, outlet and air vent of feeder are installed incorrectly, all of which would bury severe hidden troubles in running of weightless type feeder at a later date. Therefore, when installation requirement of weightless type feeder is not attained, be sure to rectify it in time and eliminate any hidden trouble to ensure stable operation of system for a long time.

The weighting system anomaly is the 2nd important reason to have the weightless type feeder constantly generate alarm faults and unable to run normally. Because whole weighting system can stably work only without external force, a majority of weighting system anomalies and faults are caused by external force.

There are usually several manifestations below:
1. Mechanical fault of weighing platform：When such fault is caused mainly by repairing or cleaning the weightless feeder，if safety device is not installed, it will cause weighing platform to suffer external force and damage. Therefore when performing above-mentioned work, be sure to operate according to operating rules so as to prevent platform from being damaged or raw material from being seized in weighing platform，thereby influencing unsteadiness of weighting system. Therefore be sure to clean up raw material on platform in time and clear all faults.

2. flexible connection out of service：Most of flexible connection materials are made of silica gel. Because flexible connection always contacts raw material. Especially when it contacts some corrosive raw materials, this would have flexible connection harden and lose function of unloading external force. So be sure to periodically clean or replace flexible connection；
3. Exhaust system blocked and operation obstructed：If exhaust system is too simple and there is only small area filter, especially if powders are not cleaned up when powders are processed, it would cause the filter to be blocked and exhaust not to be unobstructed. Be sure to periodically clean the filter or establish the better exhaust system；

4. Incorrect wiring with feeder：Because motor or encoder wire on weightless feeder would be moved incautiously in the production process to cause the weighting system to be interfered, be sure to carefully dispose wire position in time to ensure system is free from wire interference.

In a word，the above are only problems and faults recurrent in the weightless type feeder. There are many reasons to have the weightless feeder unable to normally work. The users using such equipment should carefully read operating rules to deeply understand the principle of feeder structure, constantly summarize and study to decrease operating troubles of equipment.