Development of a Rocking Piston Type Dry Vacuum Pump for ... · dry vacuum pump. As shown in the...
Transcript of Development of a Rocking Piston Type Dry Vacuum Pump for ... · dry vacuum pump. As shown in the...
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1. Introduction
Conveyance systems with vacuum suction devices are
used to convey work items in devices for mounting elec-
tronic parts at specific locations on printed circuits and
devices for testing the electric performance of packaged
electronic parts. Air ejectors are often used as a vacuum
source for these devices.
One air ejector unit, consisting of an ejector, electro-
magnetic valves that turn suction on and off, vacuum sen-
sors, filters and other parts, is required for each suction
surface. For example, in a test device for packaged ICs,
sixteen suction pads (two sets of eight units) are allocated
to each block, where �� ICs are simultaneously delivered
for testing. The amount of compressed air required for
an ejector in this example is �00 (�� x ��) NL per minute,
indicating a large consumption of compressed air. Mean-
while, how to save energy has become a hotly debated is-
sue, and in recent years more manufacturers have begun
using mechanical-type vacuum pumps for energy-saving
purposes. ULVAC KIKO, Inc. plans to develop a series
of rocking piston type dry vacuum pumps with pumping
speeds ranging from �� to ��0 L per minute to meet the
growing need to save energy. This report describes the
structure and characteristics of the DOP-��0S vacuum
pump, which offers the highest pumping speed among
vacuum pumps in this series.
2. Energy-Saving Effects in Air Ejectors and Mechanical-type Vacuum Pumps
We will start with the basic principle of air ejectors.
An air ejector is a device used to generate vacuums by
using compressed air. Figure � shows the basic principle
of operation. As the figure shows, compressed air is sup-
plied through the throttled nozzle and discharged into
the diffusion chamber at high speed, and then expands
and flows into the diffuser. As compressed air flows into
the diffuser, the pressure in the diffusion chamber drops,
causing air to flow in from the suction port (�) for dis-
charge together with compressed air through the diffuser
and exhaust port.
Figure � shows an example of an air ejector unit.
The figure shows a typical suction conveyance system
using air ejectors. This system consists of an air com-
pressor and a dryer for producing compressed air, ejec-
tor units, electromagnetic valve for air pressure supply,
vacuum breaker valve, ejector, vacuum sensor, actuator,
suction pad and other miscellaneous parts.
Assuming a suction conveyance system equipped with
air ejectors for IC test equipment as shown in Figure �,
we will estimate its annual running cost and carbon diox-
ide emissions based on the amount of air and power con-
sumed by the air compressors.
The system assumed in our estimate consists of �� air
ejector units (four sets of eight units), each of which suc-
tions air inflow at �� NL per minute and consumes com-
pressed air at �� NL per minute, with ultimate pressure of
�� x �0� Pa. We assume that suction items weigh several
grams per piece and that the system is operated for �0
days per month, eight hours a day (of which four hours
are spent for suction). The amount of compressed air re-
quired to obtain the required air inflow is �� NL per min-
ute per unit, and since we have four sets of eight ejector
units, the total amount of compressed air supply required * Research and Development Division, ULVAC KIKO, Inc.
Figure 1 Air Ejector
ULVAC TECHNICAL JOURNAL (ENGLISH) No.69E 2008
Development of a Rocking Piston Type Dry Vacuum Pump for Suction Conveyance
Hiroshi Hayashi*
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is �00 NL (� x � x ��) per minute. Given the number of
suction hours per day and operating hours per month,
the required amount of compressed air supply is �,��0
m� per month and ��,0�0 m� per year. Under these condi-
tions, the air compressors consume �.� kW of power, and
if operated for four hours per day using stagnating air, the
air compressors will consume �,��� kWh of power a year.
Assuming that power cost is �� yen per kWh, the annual
power cost will be ��,��� yen with annual carbon dioxide
emissions totaling �,��� kg (calculated based on 0.�� kg
of carbon dioxide emitted per kWh). (Of course, other
costs such as maintenance cost must also be considered.)
We also made estimates for mechanical-type vacuum
pumps. When assuming that air inflow of �� NL per
minute is produced per air ejector, past records lead us
to conclude that a mechanical-type vacuum pump with a
pumping speed of �00 L per minute will have sufficient
capability to produce air inflow equivalent to the total
amount produced by four sets of eight air ejector units,
even allowing for a �0% leeway. Therefore, the newly
developed rocking piston type dry vacuum pump (DOP-
��0S) has the capability required for the system. The
DOP-��0S consumes 0.�� kW of power and assuming
eight hours of operation a day, the annual power con-
sumption will amount to �,��0 kWh. Again assuming a
power cost of �� yen per kWh, the annual power cost will
total ��,��0 yen with carbon dioxide emissions of ��� kg
(calculated based on 0.�� kg of carbon dioxide emitted
per kWh).
The above results regarding the annual running cost
required for air ejectors and mechanical-type vacuum
pumps, and the resultant carbon dioxide emissions lead
us to conclude that using mechanical-type vacuum pumps
is likely to save energy consumption by approximately
��% compared with air ejectors, as well as reduce carbon
dioxide emissions.
3. Mechanical-type Vacuum Pumps Suited to Suction Conveyance
Chapter � described the energy-saving effects of me-
chanical-type vacuum pumps for an IC test system. This
chapter describes the various types of mechanical-type
vacuum pumps that are expected to contribute to saving
energy.
3.1 Sliding Vane Type Dry Vacuum PumpFigure � shows a photo of a sliding vane type dry vacu-
um pump (DSB series); Figure � shows its structure.
As shown in Figure �, a sliding vane type dry vacuum
pump is an mechanical-type vacuum pump consisting of
a rotor, cylinder, vanes and other parts. This pump of-
Figure 3 Sliding Vane Type Dry Vacuum Pump
Figure 2 Example of an Air Ejector Unit
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fers the advantage of not using oil on sliding surfaces,
thereby providing a clean environment, and its multi-vane
structure enables stable pumping with little vibration. The
simple structure also enables high-speed pumping in low-
vacuum areas. Conversely, the pump is disadvantageous
in that its multi-vane structure continues to slide inside
the rotor and along the inner cylinder wall, thereby gen-
erating heat by friction and increasing power consump-
tion. The friction causes the vanes to wear down quickly,
making it necessary to perform maintenance about once
every �,000 hours.
3.2 Diaphragm Type Dry Vacuum PumpFigure � shows a photo of a diaphragm type dry vacu-
um pump; Figure � shows its structure.
A diaphragm type dry vacuum pump is a mechanical-
type vacuum pump that drives air via the pumping move-
ment of a diaphragm attached to a connecting rod, which
in turn is affixed to an eccentric rotary shaft. This pump
offers several advantages such as making little operating
Figure 5 Diaphragm Type Dry Vacuum Pump Figure 6 Structure
noise, a simple structure that is easy to maintain, rela-
tively long service life, and providing a clean environment
since it needs no oil. Its disadvantage is the diaphragm’s
restricted pumping movement, which makes it difficult to
increase pumping speed.
3.3 Rocking Piston Type Dry Vacuum PumpA rocking piston type dry vacuum pump is a mechan-
ical-type vacuum pump that drives air via the pumping
movement of a connecting rod af fixed to an eccentric
rotary shaft, just like a diaphragm type dry vacuum pump.
This pump uses no oil on sliding surfaces, and thus pro-
vides a clean environment. The pump also has features
a simple, easy to maintain structure, and since it moves
with a longer stroke than a diaphragm type pump, its size
can be reduced while at the same time increasing pump-
ing speed. This pump provides ultimate pressure approxi-
mating that of a diaphragm type pump and achieves stable
performance at low pressure. Unlike sliding vane type
dry vacuum pumps, vanes do not get broken due to the
adhesion of powder caused by friction, and the abrasion
of sealing material can be easily prevented by monitoring
the pressure. Market records show that the service life of
sealing material used in a rocking piston type dry vacuum
pump is around �0,000 hours. The disadvantage of this
pump is that it generates louder operating noise than a
diaphragm type dry vacuum pump.
4. Development of a Suction Conveyance Pump Series
ULVAC KIKO, Inc. is developing a series of rocking pis-
ton type dry vacuum pumps for various systems, such as
small-size electronic parts mounters, circuit board print-
Figure 4 Structure Diagram
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top of the cylinder, along with a pump head separating the
suction chamber and exhaust chamber. A gasket and a
pump head cover are attached to the pump head.
Air evacuation movement is produced by the rotating
eccentric rotary shaft connected directly to the motor,
which causes the connecting rod to move up and down in-
side the cylinder. The connecting rod, linked to the rotary
shaft at a single point, makes a rocking movement as it
moves up and down. The connecting rod movement from
the top dead point to the bottom dead point closes the
exhaust valve and opens the suction valve, thereby allow-
ing an intake of air through the suction port. Conversely,
connecting rod movement from the bottom dead point to
Figure 8 Principle of Movement
ers, taping equipment and mechanical-type packaging
machines.
Four types of pumps with pumping speeds ranging
from �� L per minute to ��0 L per minute have been de-
veloped, with power source specifications appropriate for
overseas use.
Figure � shows the lineup of our rocking piston type
dry vacuum pumps; Table � lists the major pump specifi-
cations.
5. Principles of Rocking Piston Type Dry Vacuum Pumps
This chapter describes the structure and principles of
rocking piston type dry vacuum pumps.
Figure � shows the structure of a rocking piston type
dry vacuum pump. As shown in the figure, a rocking pis-
ton type dry vacuum pump has an eccentric rotary shaft
around the motor shaft, with a bearing and a connecting
rod affixed to the rotary shaft. A sealing disc made of spe-
cial resin (called cup packing) is affixed to the upper part
of the connecting rod, in contact with the inner cylinder
wall. A suction valve and exhaust valve are installed on
Figure 7 Development of a Series of Rocking Piston Type Dry Vacuum Pumps
Table 1 Major Pump Specifications
Model namePumping speed
(L/min)Ultimate pressure(x 103Pa)50Hz 60Hz
DOP-90S 85 95 19.0DOP-180S 170 190 19.0DOP-300S 300 330 8.0DOP-420S 420 460 17.3
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6.2 Characteristics1) Three options are available for choosing the loca-
tion of the suction port in line with the layout of system piping.
When installing a pump in a system, it is often neces-
sary to install the pump in an area where electric parts
and pipes are concentrated, necessitating such adjust-
ment at installation as attaching a special connector to
the suction port according to the layout of system pip-
ing. The newly developed pump is designed to provide
three options for the suction port location in order to
match a wider range of layouts and facilitate piping in-
stallation.
2) The structure was redesigned to improve maintain-ability.
Pumps developed in the past were designed with a
motor set between compression chambers, as shown in
Figure ��. Due to this structure, joints and pipes were
needed to connect the chambers, but then had to be
removed at maintenance and for replacing consumable
parts like O-rings. To avoid this burden, we redesigned
the arrangement of the motor and compression cham-
the top dead point closes the suction valve and opens the
exhaust valve, thereby discharging air from the exhaust
port. This cycle is repeated according to rotation of the
motor.
6. Structure and Characteristics of the DOP-420S
6.1 Internal Structure of the DOP-420S PumpFigure � shows the internal structure of the DOP-��0S
pump.
This pump consists of (�) a motor, (�) casing, (�) ec-
centric rotary shafts, (�) connecting rods, (�) cylinders,
(�) suction valves, (�) exhaust valves, (�) pump heads, (�)
gaskets and (�0) pump head covers, with four compres-
sion chambers horizontally interconnected. Piping con-
necting these chambers is incorporated into the structure
of the casing and pump head parts. Thus, the pump is
designed to eliminate connecting pipes and joints so as to
reduce the number of parts and improve maintainability.
Figure 9 Internal Pump Structure
Figure 10 Appearance
Figure 11 Parallel Movement
Figure 12 Counter Movement
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bers as shown in Figure �� to eliminate connectors and
pipes, thereby improving maintainability and reducing
the number of parts, including connectors and pipes.
3) The pump is designed to produce counter piston movement so as to reduce vibration.
As shown in Figure ��, piston pumps developed in
the past were designed so that as connecting rods �a
and �b on the left move in the arrow-indicated direc-
tion, and connecting rods �a and �b on the right move
in the opposite direction, thereby making it necessary
to attach balance weights to reduce vibration caused by
such movement. In contrast, the DOP-��0S (Figure ��)
is designed so that connecting rods ��a and ��b each
move in directions away from the motor shaft, while
connecting rods ��a and ��b each move in directions
toward the motor shaft, thereby canceling out vibration
to achieve a low level of vibration.
7. Performance Characteristics of the DOP-420S
7.1 Pumping Speed CurveFigure �� shows the pumping speed curve of the DOP-
��0S.
Pumping speed decreases as the suction pressure
drops. Although the amount of free air leaking through
the cup packing remains constant regardless of changes
in suction pressure, the effect of this incoming air be-
comes more evident as the suction pressure drops—the
reason why pumping speed decreases. The decrease is
also caused by a dead volume at the top dead point of the
piston. This does not pose any serious problem with the
newly developed pump, which is designed for the purpose
of suction conveyance.
7.2 Power ConsumptionFigure �� shows the power consumption of the DOP-
��0S.
Power consumption is at the lowest level when ultimate
pressure is achieved, reaching a peak around �0 x �0� Pa
where mechanical loss and pressure loss are greatest.
7.3 Comparison of Noise Levels with a Sliding Vane Type Dry Vacuum Pump
Figure �� compares noise levels between the DOP-��0S
and a sliding vane type dry vacuum pump (at �0 Hz).
With a rotary dry vacuum pump, the noise level shows
a moderate increase from ultimate pressure to �0 x �0� Pa,
subsequently showing a gradual decrease until �0 x �0�
Pa, and then increasing again toward the level of air pres-
sure. This is caused by an increase in airflow noise.
With the DOP-��0S, the noise level is lowest when ulti-
mate pressure is achieved, remaining almost flat after the
pressure exceeds �0 x �0� Pa. Compared with the sliding
vane type dry vacuum pump, the noise level is consistent-
ly lower by � to � dB (A).
Figure 15 Noise Level
Figure 13 Pumping Speed Curve
Figure 14 Power Consumption
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8. Service Life of Consumable Parts and Maintainability
8. 1 Service Life of Consumable PartsThe service life of consumable parts required for the
rocking piston type dry vacuum pump largely depends
on that of the cup packing (sealing material) that slides in
contact with cylinders. Cup packing is normally made of
fluoride resin and the matching parts are made of anode-
oxidized aluminum or stainless steel. The same standards
for the DOP-�00S (developed earlier) concerning ma-
terials for cup packing (related to consumables) and its
matching parts, sliding distance and rotational velocity
were adopted for the design of the DOP-��0S. Figure ��
shows measurements taken every �,000 hours to evaluate
the stability of ultimate pressure of the DOP-�00S, which
served as standards for the DOP ��0S. Pumps were oper-
ated under two different conditions: continuous operation
under ultimate pressure and operation under variable
pressure. The criterion for ultimate pressure of the DOP-
�00S was less than � x �0� Pa. With the pump operated
continuously under the variable pressure condition, the
pressure began rising after about ��,000 hours, exceed-
ing the criterion after about ��,�00 hours. In contrast,
with the pump operated continuously under the ultimate
pressure condition, the pressure began rising after about
��,000 hours, exceeding the criterion after about �0,000
hours. When the criterion was exceeded, the cup packing
in contact with the cylinder had lost about ��% of its initial
weight due to abrasion. These results lead us to conclude
that the appropriate service life for cup packing is about
��,000 hours.
The DOP-��0S has been operated for more than �,000
hours without any increase in ultimate pressure. Cup
packing lost about �0% of its initial weight on the surface
in contact with the cylinder after �,000 hours of operation,
which makes it likely that it will last about ��,�00 hours
before its weight falls to the same level as that of the DOP-
�00S.
8.2 MaintainabilityAs Figure �� shows, the new vacuum pump is designed
so that cup packing (sealing material)—a consumable—can be easily and quickly replaced without using special
tools by simply loosening the hexagon socket cap screws
Figure 17 Replacement
Figure 16 Stability of Ultimate Pressure
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used to affix the pump head and cup packing retainers.
9. Summary
Since about five years ago, we have been recommend-
ing that the system manufacturers of electronic parts
mounters and IC test equipment use mechanical-type
vacuum pumps to save energy, but few manufacturers
have accepted our proposal because its initial cost would
be high. Nevertheless, more system manufacturers have
begun using our series of rocking piston type dry vacuum
pumps over the past two years. We believe that this is due
to the growing demand for saving energy among equip-
ment end users.
A mechanical-type vacuum pump requires a structure
connecting a mobile pump head attached with a suction
pad to the pump via movable vacuum piping, and since air
is repeatedly evacuated with pressures ranging from �00
to �0 x �0� Pa, the air flow within the vacuum piping rang-
es between turbulent and laminar flows. Consequently,
there are areas where ordinary conductance calculations
as used in vacuum technology do not apply. System manu-
facturers must develop the necessary know-how and skills
regarding piping. Our company provides various services
to help choose pumps of different types, including per-
forming tests using actual production machines.