Home Page T-42 flanges  (see  the  special  section  dealing  with  drive  alignment).    Pulley  materials  and  dimensional accuracy  also  influence  drive  noise.    Some  users  have  found  that  steel  pulleys  are  the  quietest, followed closely by aluminum.  Polycarbonates have been found to be noisier than metallic materials. Machined pulleys are generally quieter than molded pulleys.  The reasons for this revolve around material density and resonance characteristics as well as dimensional accuracy. 9.5   Static Conductivity Small synchronous rubber or urethane belts can generate an electrical charge while operating on a drive.  Factors such as humidity and operating speed influence the potential of the charge.  If determined to be a problem, rubber belts can be produced in a conductive construction to dissipate the  charge  into  the  pulleys,  and  to  ground.    This  prevents  the  accumulation  of  electrical  charges that  might  be  detrimental  to  material  handling  processes  or  sensitive  electronics.    It  also  greatly reduces the potential for arcing or sparking in flammable environments.  Urethane belts cannot be produced in a conductive construction. RMA  has  outlined  standards  for  conductive  belts  in  their  bulletin  IP-3-3.    Unless  otherwise specified,  a  static  conductive  construction  for  rubber  belts  is  available  on  a  made-to-order  basis. Unless otherwise specified, conductive belts will be built to yield a resistance of 300,000 ohms or less, when new. Nonconductive belt constructions are also available for rubber belts.  These belts are generally built specifically to the customers conductivity requirements.  They are generally used in applications where one shaft must be electrically isolated from the other. It  is  important  to  note  that  a  static  conductive  belt  cannot  dissipate  an  electrical  charge through  plastic  pulleys.    At  least  one  metallic  pulley  in  a  drive  is  required  for  the  charge  to  be dissipated to ground.  A grounding brush or similar device may also be used to dissipate electrical charges. Urethane  timing  belts  are  not  static  conductive  and  cannot  be  built  in  a  special  conductive construction.    Special  conductive  rubber  belts  should  be  used  when  the  presence  of  an  electrical charge is a concern. 9.6   Operating Environments Synchronous  drives  are  suitable  for  use  in  a  wide  variety  of  environments.    Special considerations may be necessary, however, depending on the application. Dust:  Dusty environments do not generally present serious problems to synchronous drives as  long  as  the  particles  are  fine  and  dry.    Particulate  matter  will,  however,  act  as  an  abrasive resulting in a higher rate of belt and pulley wear.  Damp or sticky particulate matter deposited and packed into pulley grooves can cause belt tension to increase significantly.  This increased tension can  impact  shafting,  bearings,  and  framework.    Electrical  charges  within  a  drive  system  can sometimes attract particulate matter. Debris:    Debris  should  be  prevented  from  falling  into  any  synchronous  belt  drive.    Debris caught in the drive is generally either forced through the belt or results in stalling of the system.  In either case, serious damage occurs to the belt and related drive hardware. Water:  Light and occasional contact with water (occasional wash downs) should not seriously affect synchronous belts.  Prolonged contact (constant spray or submersion) results in significantly reduced tensile strength in fiberglass belts, and potential length variation in aramid belts.  Prolonged contact  with  water  also  causes  rubber  compounds  to  swell,  although  less  than  with  oil  contact. Internal belt adhesion systems are also gradually broken down with the presence of water.  Additives to water, such as lubricants, chlorine, anticorrosives, etc. can have a more detrimental effect on the