August 17, 1998 Design News
BREAKTIME
The lighter side of engineering
New pulley no 98-pound weakling
Paul Osenkarski Engineering Project
Manager
Life Fitness
Franklin Park, IL
Franklin Park, IL--It's no secret that today's
society is fixated on becoming physically fit. Given
this frenzy, any given piece of exercise equipment undergoes
hours of abuse, which the machine must be able to endure.
At Life Fitness we design and manufacture computerized
fitness products, such as cardiovascular and strength
training equipment, for use in health, fitness, and
wellness facilities. One of our main design considerations:
reliability.
Test bed for improving
exercise equipment
Using the following design alternatives,
engineers improved the pulley design in the
Life Fitness system.
|
|
Average Torque (ft-lbs)
|
Failure Mode
|
Existing Pulley Design (20% glass-filled
polypropylene, six gates)
|
200.5
|
Fracture at outer and inner knit lines
and spoke base
|
Zytel material, six gates
|
305
|
Fracture at outer and inner knit lines
|
Same material, three gates closed
|
162.5
|
Fracture at spoke base
|
Zytel material, three gates closed
|
271.7
|
Fracture at spoke base
|
With this in mind, we looked to redesign the pulley
used in the Life Fitness HR 9500 Cross-Trainer--Total
Body System. It translates the machine's movements to
rotary motion that drives a belt linked to an electrical
alternator. In turn, the alternator creates and controls
resistance to movement of the machine's pedals and arms.
The Cross Trainer is used by exercise-goers seeking
a total body workout with virtually no impact. Pushing
and pulling arm motions maximize upper body benefits,
while forward and backward elliptical pedal motion targets
the lower body. Users can choose 20 resistance levels,
and the system's Heart Rate Zone training program automatically
adjusts resistance according to the user's heart rate.
The 20% glass-filled polypropylene we molded the pulley
from did not provide the torque-strength characteristics
we required. We needed either a new material or part
redesign that would prevent the pulley from fracturing
during operation and would meet the following specifications:
-
140 inch-lbs of torque at 600 rpm
-
8.15 inches pitch diameter
-
A minimum of 10,000 fully functioning operating
hours
We decided to test four design alternatives by changing
the material from 20% glass-filled polypropylene to
DuPont's Zytel glass-bead reinforced nylon, the gating
pattern to three gates instead of six, or a combination
of these. The tests were as follows:
-
Change neither the material or gating pattern
-
Change the material, but not the gating pattern
-
Use the same material, but change the gating pattern
-
Change the material and gating pattern
During the torque tests we placed the pulley on a flat
plate with tapped holes positioned on either side of
the spokes. Screws and washers that trapped the spokes
to the plate held the pulley in position. We used a
modified shaft to apply the torque load by welding a
pin onto the shaft to fit into the slot of the pulley's
metallic hub. The shaft applied a torque load to the
center of the pulley, while the outer edge was constrained
on the plate.
The pulleys made of Zytel FE5327 demonstrated more
than 50% higher resistance to torque failure than the
previous glass-filled polypropylene. The material helps
minimize belt wear, while providing adequate stiffness
and strength.
As a result, we made no changes to the molded part
design except to accept different hubs. More importantly,
Zytel nylon provides the torque strength we needed to
avoid pulley fracture during operation.
Design Objective
Improve exercise equipment to endure the abuse of exercise
fanatics.
Identify a pulley material or design change that:
Zytelr nylon resin from DuPont
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